From Pathfinder Seeker of Secrets
The feat Boon Companion:
Prerequisites: Animal companion or familiar class feature.
Benefit: The abilities of your animal companion or familiar are calculated as though your class were 4 levels higher, to a maximum effective druid level equal to your character level. If you have more than one animal companion or familiar, choose one to receive this benefit. If you lose or dismiss an animal companion or familiar that has received this benefit, you may apply this feat to the replacement creature.
Special: You may select this feat more than once. The effects do not stack. Each time you take the feat, it applies to a different animal companion or familiar.
The effect of the feat isn't coded.
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What exactly is or isn't happening?
When you take the feat, it should give you a new selection (Choose Familiar or Animal Companion). After you take the selection, the results should take place. however, my limitation is the "Choosing" which of 20 animal companions, that isn't easy to deal with.
The animal companion (didn't tried it with familiar yet) doesn't get the extra companion levels.
And i should have been more thorough with my tests, it doesn't work when you get an animal companion via the animal domain.
I checked boon companion with animal companion from ranger's hunter's bond and it works perfectly.
I checked boon companion with animal companion from druid's nature bond and it works perfectly.
I checked boon companion with animal companion from sorcerer's animal companion bloodline power (sylvan bloodline) and it does NOT work.