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An HM slave, or sometimes HM mule, is a Pokémon which is in the party not to be used in battle, but to provide the use of HM moves or other moves usable outside of battle that typically are not suitable for a powerful team. An HM slave itself is usually considered to be a Pokémon that can have all four of its moves taken up by HMs, and occasionally by any move usable outside of battle.
Most HM slaves are incredibly common Pokémon, typically those which can be captured on the first routes of a given game. A majority of them are Template:Type2, which is fitting, considering that this type is shared with three of the eight current HMs, and Rock Smash, though it is Fighting, is also usually available to them as well. Template:Type2 Pokémon are also common, as they also have three HM moves which share their type, and many are also able to use at least one or two of the other five. Because of these two tendencies, as well as its typing, Bibarel is often regarded as the ideal HM slave.
Due to both Fly and Defog sharing the Template:Type2, Pokémon of this type are also considered to be good HM slaves in Generation IV, especially considering Defog's effect does nothing to harm the opponent directly at all (while Flash lowered the opponent's accuracy, causing moves to sometimes miss, Defog merely lowers their evasiveness). HM slaves like Tropius and Pelipper are ideal for these roles since they are capable of using other HM moves along with Fly and Defog.
Typically, legendary Pokémon like Lugia and most especially Mew can learn many, if not all, HM moves. Because their high stats mean they are incredibly useful for battle, most consider it a waste to put any HM moves, aside from perhaps Surf or Waterfall, on a legendary, especially an event legendary like Mew, available only occasionally directly from Nintendo itself.
For Generation III, an example of two Pokémon learning all eight of the HMs and are available in the Hoenn region are Tropius (Cut, Fly, Strength and Rock Smash) and Psyduck (Surf, Flash, Waterfall and Dive).
Most compatible HM slaves for each generation