Slowbro is a bipedal, pink Pokémon with a tan, striped belly and a tan muzzle. It has large, vacant eyes, curled ears, and two pointed teeth protruding from its upper jaw. On its hands, it has three clawed fingers, and two clawed toes on its feet. There is a Shellder attached to its tail. This Shellder has a gray, spiraled shell with darker gray spots. There are several spikes across the surface of the Shellder, and it holds onto Slowbro with many sharp teeth. If the Shellder is knocked off, Slowbro will revert to its unevolved form.
Slowbro has lost the capability to feel pain due to the Shellder's poison. However, it apparently becomes inspired when the Shellder bites down on its tail. While it is capable of unleashing powerful psychic attacks, Slowbro is typically rather peaceful and usually prefers not to fight. Because it can no longer fish using its tail, it instead swims to catch its prey and allows the Shellder to feed on its leftover scraps. Slowbro tends to live near ponds, lakes, and seashores.
When Slowbro Mega Evolves into Mega Slowbro, it becomes engulfed by the Shellder to the point where only its head, arms and tail are visible. The spiral Shellder now has an opening at its tip, allowing the tail of Slowbro to stick out, which it balances on due to its legs being engulfed. It can move by bouncing, using its tail as a spring. The spikes on the Shellder are now reduced to four on the bottom section of its body, with the dark spots only visible on its middle section. Mega Slowbro's energy has gathered into Shellder, which has increased its hardness. This allows it to act as an armor so hard that nothing can scratch it.
Slowbro's first real appearance was under the ownership of Lorelei in Playing Horsea where many were used for her Ice-type army. With the Shellder clamped onto its tail, Lorelei explains that her Slowbro has sharpened senses and is capable of looking in all directions, thanks to the extra pair of eyes, allowing it to discover Green's ploy using Horsea.
Despite it being a commonly known and accepted fact that Shellder triggers its evolution, mentioned in most of its Pokédex entries, Slowbro's evolution from Slowpoke has nothing to do with Shellder in the core series, unlike how the evolution of Mantyke requires a Remoraid.
Many Pokémon are noted to evolve by two or more Pokémon combining, such as Metang being formed by two Beldum. However, the anime does not show this and simply spawns the second Pokémon out of nowhere: an example is Morrison's Beldum, which simply evolved into Metang without the need of joining to another Beldum. The evolution of Slowpoke into Slowbro or Slowking is the only time that evolution by two Pokémon physically merging is shown.
Slowbro's shell and lifestyle are somewhat similar to hermit crabs, though it is physically similar to giant otters, giant salamanders, and hippopotamuses. Slowbro's appearance may also be a reference to the mythical sazae-oni, a magical sea demon of Japanese folklore distinguished by its spiny "sazae" or "turban shaped" snail shell.
Its parasitic relationship with Shellder may be inspired by leeches. Interestingly enough, Slowbro is known as the Hermit Crab Pokémon and many real-world parasites, including leeches, often target crabs.
Slowbro is a combination of slow and bro (short for brother). It may just be used in a general sense, or it could refer to the brotherly/symbiotic relationship shared by the combined form of Slowpoke and Shellder.
Yadoran may be taken to mean 宿らん (to not lodge/dwell). It may also reference 宿借り yadokari (hermit crab) or 宿主 yadonushi ([parasitic] host).