I've found atleast five so far.
Most of them you have to defeat them in combat to unlock them
Giant rat (got to defeat the giant rat in the sewer to unlock)
Lime Slime ( got to find a lime slime in the cave east of town and give them a potion and they can be found outside once you leave, will be the only friendly one in the cave but randomly spawns
Barrel Master: Tip over the big barrel by the gate
Chimera: Found down by the docks
Raving Fan: Defeat their former boss (can easily be found when looking for the left arm)