
Reksew_Trebla |
The concept is that the spellcaster befriended a spirit (roleplay it being the same summon each time), and the casting of spells to summon it is essentially an invitation which it accepts, and it takes the form it believes will best suit the expected day's events (within the limits of its abilities, which grow with the caster). Most days are out of combat more than they're in combat, so a fuzzy mammal that can have its ears scratched is usually the better choice over something like a fire elemental that inadvertently sets everything ablaze.
Uh, hate to be the bearer of bad news, but if that was the purpose, then you just lost the debate on it being overpowered, because Paizo already has an option for the summon of a spirit that is the same spirit everytime, except it can’t change to a different summon option, making your idea overpowered in comparison.
Feat: Summon Guardian Spirit
Template for feat: Guardian Spirit

ErichAD |

Dude, he found an alternative, there's no reason to ride him on it or try and rub it in his face. His intended use sounds like it wouldn't have been overpowered, not everyone is familiar with how certain things can be broken without looking into it.
As an additional alternative, the Totem Spiritualist gives you an animal companion/phantom mashup that gives you additional animals to choose from as you gain levels. And you can store it in your head when you aren't using it, so it's pretty convenient. But they're phantoms so I'm not sure how well they fill the role of friendly pet. Another problem with them is it's unclear how the class stacks with other levels that grant an animal companion.
There's also the option of using an unsworn shaman and selecting nature. You should be able to reselect the specifics of the animal companion version of your familiar when you do so. But that's an 18th level thing, so you've went through most of the game without the main concept in place.