This trainer perk functions like the druid's animal companion ability (part of the nature bond class feature) except as noted here.

• The trainer’s effective druid level is equal to his trainer level –3 (minimum 1)
• A trainer who selects an animal companion can choose one animal he has captured that meets the requirements to be an animal companion for his effective Druid level. While active, that animal becomes the trainer’s animal companion. They share the trainer’s favored enemy and favored terrain bonuses (if any).

When the monster trainer gains a new level in any class that gains an animal companion, he can update his animal companion or choose a new animal he has captured that meets the requirements to be an animal companion for a druid of his effective level.