The Leadership feat allows a character to gain a loyal cohort. With the GM’s approval, this cohort can be a similarly aligned monster rather than a humanoid with the appropriate number of class levels.
Monsters on the following list all work well as cohorts (be they assassins, bodyguards, mounts, etc.)—their effective cohort “level” corresponds to the level available to the PC as afforded by his Leadership score.
A monster cohort gains experience points as if it were a character of its cohort level, and when it gains enough XP to advance a level, it should generally gain a key class level (fighter is often the best choice for most cohorts). You can use the monsters on the following table as guidelines when determining effective cohort levels for monsters not on this list.