HM Slaves are Pokémon used for the sole purpose of using HM moves when other Pokémon can't learn them, or they are unwanted in a Pokémon's moveset. Chuggaaconroy has used several of these in his Let's Plays.
Diglett was caught in Episode 11. She was used for Cut and Dig. She was no longer used after Slave was obtained.
Slave was caught in Episode 12. He is a Paras and was used for Cut and Flash. He received his name in Episode 17.
Slave's other moves are Scratch and Stun Spore.
In Episode 22 he was used to fight Koga's Weezing but was KO'ed before he had a chance to do anything.
Slave Mk 2
Slave Mk 2 was caught in Episode 44. It is a Tentacool and was used for Waterfall.
Onix was caught in Episode 51. He was used for Rock Smash.
Onix's other moves were Dragonbreath, Slam, and Iron Tail.
Hoothoot was caught in Episode 3.