From Bulbapedia, the community-driven Pokémon encyclopedia.
A multiple entity Pokémon is a single Pokémon creature consisting of two or more separate living entities. It may be multiple Pokémon that have fused together in one way or another in order to evolve, or simply a group of entities that have been connected since birth. This strange phenomenon is relatively rare, occurring only with a small percentage of Pokémon species.
Characteristics of multiple entity Pokémon
As a general rule, the separate entities which make up the Pokémon can think as one, planning battle strategies and synchronizing their movements as if they were a single organism. This may not always be the case, however; sometimes the entities are still considered separate Pokémon in their own right, and in some cases, will occasionally disconnect and return to being two separate living creatures.
List of multiple entity Pokémon
Paras and Parasect
Paras removing its tochukaso
Paras are insectoid creatures that have parasitic tochukasu mushrooms growing on their backs. Their relationship is somewhat symbiotic—the mushroom leeches most of the nutrients obtained, while the Paras uses its spores for attacking. While the mushrooms can be removed, scientists believe doing so is bad for the Paras's health.
After Paras evolves into Parasect, the mushrooms are larger than the host. The fungus completely controls the Pokémon and does all the thinking. In fact, Parasect frequent dark, damp places only because of the parasite's preference.
A Dugtrio is comprised of three Diglett huddled together
When three Diglett gather in one place, the Pokémon will evolve into a Dugtrio. It will not undergo a very drastic change in evolution, though each Diglett which makes up the Pokémon will be noticeably larger, and will generally be wearing a frowning expression. The three Diglett triplets think in exactly the same way, and work together to dig large tunnels and trigger earthquakes.
As no-one has ever seen the lower half of a Dugtrio's body, which is concealed under the ground, it is unclear whether the three Diglett triplets are fused together or if they are still separate bodies. Some Pokédex entries state that Dugtrio are triplets that emerged from a single body, implying that the fusion of three Diglett is not always necessary for the creation of a Dugtrio (as is the case in the games).
Slowbro and Slowking
After waiting patiently with its tail dipped in the water, a Shellder will eventually latch onto the Pokémon's tail and trigger evolution
Slowpoke emit a sweet-tasting substance from their tails, which acts as a lure for Shellder. When a Shellder bites onto a Slowpoke's tail, the two Pokémon undergo a major transformation, evolving into a Slowbro. Slowpoke will begin to walk upright, and the Shellder will become a spiral-shaped shell instead of a bivalve, with noticeably different facial features. The Shellder will latch onto Slowbro's tail, following it wherever it goes. However, occasionally it is possible for a Shellder to let go of the tail, in which case Slowbro will revert to its original Slowpoke state.
On rare occasions, after being attracted by a King's Rock, a Shellder will bite into a Slowpoke's head instead of its tail. When this happens, Slowpoke will instead evolve into a Slowking. Poison from Shellder's teeth seeps into Slowking's head, unlocking an extraordinary power and intellect. Thus, Slowking are a lot more intelligent than Slowpoke and can use their Psychic abilities to full potential. It is also capable of learning to speak like a human, an ability which only a select few Pokémon have mastered.
Magneton and Magnezone
Magneton is comprised of three Magnemite, linked by strong magnetism
Magneton is the result of three Magnemite linking together. The three Magnemite are held together by a magnetic force so strong that it can cause headaches to those nearby. A Magnemite is also capable of "generating" two extra Magnemite in order to evolve if there are no others in the vicinity.
There are many ways that the three Magnemite which make up a Magneton can be connected, though most arrange themselves with one Magnemite on top and two below, creating a triangular formation. In some cases the Magnemite are not actually attached one another, but are still kept together by their magnetism.
When a Magneton is exposed to the magnetic field at Mount Coronet, it will evolve a second time into a Magnezone. One of the three Magnemite becomes more dominant and grows dramatically in shape and size, becoming more saucer-shaped and sporting a large red eye and yellow antenna on top of it. The other two Magnemite are linked permanently to either side of the main Magnemite. Magnezone has three magnet limbs, seemingly one from each of the original Magnemite. Two are used as limbs while the third trails behind, acting as a "tail" of sorts. While Magnezone is slower than Magnemite and Magneton due to its additional weight, its attack power is much greater.
Doduo and Dodrio
With its two heads, Doduo is perhaps one of the most bizarre of all Pokémon species
Doduo's most striking feature is its two heads, and thus two brains. These two brains are said to be identical, though in rare cases they may differ from each other. As the two heads are connected the same body, they can communicate telepathically, sensing each other's feeling and emotions. They are also skilled in cooperation; Doduo can often be seen alternatively raising and lowering its two heads to maintain balance whilst running. Doduo has also developed a habit of keeping one of its heads awake while the other sleeps, allowing it to keep a watchful eye on its surroundings and potential dangers at all times.
Dodrio's size and speed make it a popular form of transport
Things become even more complex upon evolution to Dodrio, however, with the addition of a third head. Each of Dodrio's heads has a very different personality; it is said to have one for joy, one for sorrow and a third for anger. This clash of personalities can often lead to disagreement and disputes among the three heads. However, if one of the heads is eating, the other two will also be satisfied; this is further evidence of a connection between them. When there are not quarreling, Dodrio's heads will work together to devise complex battle strategies.
The heads are not the only thing Dodrio has three of, however. It also has three sets of hearts and lungs, allowing it to run for great distances without stopping to rest. It also has three kidneys.
Exeggcute and Exeggutor
A single Exeggcute, consisting of six separate entities
While an Exeggcute may appear to be six individuals of the same species, it is in fact a single Pokémon creature. Despite this however, each "egg" which makes up an Exeggcute seems to have its own sense of identity. Being part Psychic-type Pokémon, they share a close telepathic bond, communicating in wavelengths only they can interpret. Although an Exeggcute is technically a small colony in itself, multiple Exeggcute, each consisting of six eggs, will sometimes form much larger colonies, with each Exeggcute building its own private nest to house its six members.
Exeggute and Exeggutor are part of a very unusual life cycle
Each Exeggcute has a special energy bond which holds its six units together, and causes them to attract and spin around each other. If one of the eggs is ever separated from the group, this energy will quickly rejoin it with the others.
Exeggcute evolves into Exeggutor using a Leaf Stone. Exeggutor also has multiple heads, though normally only has three compared to the six it had as an Exeggcute. If it lives in a good environment, however, it will grow more heads. Each head thinks independently, but unlike Dodrio, they are friendly and never disagree with each other.
If one of Exeggutor's heads falls off, it will emit a telepathic call, searching for five other individuals with which to form a new Exeggcute, thus completing the life cycle.
Weezing is the result of two Koffing slowly fusing together, over a period of many years. This is unique in the Pokémon world, as most Pokémon evolve in a matter of seconds. This is not the only way a Weezing can be created, however. Similarly to Magnemite, a Koffing can also "generate" the other Koffing to become a Weezing, a method that is much faster to accomplish.
The mixture of poisonous gases from both of the Koffing makes this Pokémon much more dangerous after evolution. The twins are inversely affected by each other's actions; if one inflates, the other will deflate. It uses this technique to mix toxic gases inside its body.
Although Weezing are usually comprised of twin Koffing, on very rare occasions triplets have also been found.
Wynaut and Wobbuffet
Girafarig is a giraffe-like Pokémon, with the back end of its body under the control of a brain on its tail. Its tail has a face which has a brain which thinks separately from the main brain, allowing it to keep full watch of its surroundings.
Metang and Metagross
Metang is the product of a magnetic polymerization of two Beldum. This causes it to have double the brainpower of a Beldum. Beldum's "feet" become "arms," allowing Metang to perform more moves than just Take Down.
Metagross is created in the same way, with two Metang becoming magnetically fused. As with, Metang, the two heads combine, doubling brainpower (Metagross's base IQ is quadruple of Beldum). However, Metagross's internal computer systems and original organism can help generate more brainpower. Metang's "arms" become "legs" for Metagross, but as Metagross can levitate and use powerful Psychic attacks, so they also form as "arms" and "stands."
May it be noted that Metagross, aside from being the product of a fusion of 2-Metang/4-Beldum, is a multiple entity Pokémon in its own right. The Metagross that we see is mainly a metallic exoskeleton that functions in symbiosis with a smarter, smaller organism that functions as its Psychic "command center." However, this may be pure fan speculation.
Combee combined into a wall
Combee are Pokémon that are composed of three small segments, each with their own face. Sometimes Combee will join together with more Combee, and create a wall to protect their Vespiquen.
Cherubi, the cherry Pokémon, is a Grass-type that eats and gains energy through photosynthesis, using the chlorophyll in its leaves. As it cannot use all of the energy at once, it has created another "cherry," which has developed traits of life and personality such as those of the original Cherubi. However, its main purpose is a backup powersource. The main Cherubi can drain from it at any time, causing it to be smaller and more lethargic, hence its continually "sleepy" face.
Cherubi's evolved form, Cherrim, is comprised of only a single entity. This makes Cherubi one of the only true multiple entity Pokémon to become a single entity Pokémon after evolution.
Spiritomb is a manifestation of over 108 souls of sinners and wrongdoers. They were collected in their spiritual form by a magi and confined to an Odd Keystone. By the power of its souls, Spiritomb can create a ghostly "face," allowing it to perform Ghost- and Dark-type moves. However, it is confined to its keystone by the curse of the spell. All spirits comprising a Spiritomb work together to accomplish the same goal.
Vanilluxe is a Pokémon that resembles a banana split. It has two heads, each with a face. Both faces are alike, though one has a straw-like appendage that puffs clouds of snow.
Klink, Klang and Klinklang
Klink are made from two identical, gear-like bodies. They spin around to generate the energy necessary for survival. Klang can also fire away smaller gears, but it is dangerous if they do not return. Klinklang's red core acts as an energy tank, whose generated energy can be fired through Klinklang's spikes.
Zweilous and Hydreigon
Zweilous are two-headed dragons. Although they seem to have no eyes, it is unconfirmed whether they are actually blind. The two heads dislike each other, and they often create food contests.
Hydreigon, a brutal and destructive Pokémon, attacks anything that moves. Despite its three heads, it has only one brain, which is located in the center head; the heads on Hydreigon's arms are brainless.
Other instances of Pokémon combining, separating or in symbiosis
Zapmolcuno, whose official English name is nonexistent, is a fusion of the three legendary birds: Articuno, Zapdos, and Moltres. Red's Gym Badges triggered Team Rocket's Badge Energy Amplifier, which created the fusion of the legendary birds. After Zapmolcuno was defeated, the Articuno, Zapdos, and Moltres separated into their natural forms.
Kangaskhan is a Pokémon that carries its young in its pouch, resembling a kangaroo. Their babies spend most of their time in the mother's pouch, though they leave as they mature.
Mantine is sometimes shown with a Remoraid attached to its left fin.
While evolving into Ninjask, Nincada sheds its shell. Under certain conditions, the shell will become Shedinja.
A Vespiquen carries Combee in the underside of its expansive abdomens for using Attack Order, Defend Order, and Heal Order.
Probopass's "Mini-Noses" can detach and become Nosepass.
Black Kyurem and White Kyurem
Kyurem's alternate Formes are achieved by using the DNA Splicers to combine it with Zekrom or Reshiram. Black Kyurem is the fusion of Kyurem and Zekrom, while White Kyurem is the fusion of Kyurem and Reshiram. The DNA Splicers can be used again to undo the fusion. The 15th movie seems to imply that the consciousness of the resulting fusion is exclusively Kyurem's.