Not to complicate an already complicated ability but...
Would the Nature shamans true spirit ability qualify as a "class feature that functions as the druid animal companion ability"?
A bit of strange wording for sure, but I can see it both ways. I was just curious what the forums thought.
Companion Animal (Su): The shaman’s spirit animal takes the form of an animal companion of her choice, using her shaman level as her effective druid level. The animal retains all the special abilities and the Intelligence score of the spirit animal, but also has the statistics and abilities of an animal companion. If the animal is dismissed, is lost, or dies, it can be replaced in the same way as a normal spirit animal.
Yes, I'd say that it does. The way the Monstrous Companion feat's prerequisites are worded make the feat unusually flexible in application.