From Bulbapedia, the community-driven Pokémon encyclopedia.
An HM slave, also referred to as an HM mule, is a term for a Pokémon who is not trained for battle and instead stays on a player's team to use HM moves outside of battle to traverse the field. This is due, for the most part, to the fact that many of the moves learned through HMs deal little damage or have no worth on a powerful team of Pokémon.
Most HM slaves are common Pokémon, particularly those which can be captured early and can be taught many of the HM moves. This often falls into the hands of several extremely common Normal-type Pokémon, since the moves Cut, Strength, Flash and Rock Climb are their type, and they can usually learn the Fighting-type HM move Rock Smash as well. Water-type Pokémon are also common HM mules due to the appearance of such HMs as Surf, Whirlpool, Waterfall and Dive. Because of these two trends, Bibarel is often considered the ideal HM slave by many fans.
Flying-type Pokémon also sometimes act as HM slaves, since the move Fly is possibly the most useful move for travelling and, while a better combat move than several other HMs, is not always desired on a trained fighter. This became more common with the introduction of Defog in Generation IV, which is considered by fans to be the least useful HM move in combat to date.