One of my fellow players posed a scenario that I can't really find a satisfactory answer to. A level 10 character has the Wild Cohort feat and can have an animal companion as a level 7 druid. Since he can have an animal companion with 7 hit dice, can he choose to add a template instead of adding a hit die to avoid the maximum number of hit die his companion can have?
The feat Wild Cohort provides no option for the wild cohort to have a template
The benefit of the feat Wild Cohort says, in part, that
You gain an animal cohort.... Provided the DM gives her approval, at 1st level you can choose from a badger, camel, dire rat, dog, riding dog, eagle, hawk, horse (light or heavy), owl, pony, snake (Small or Medium viper), or wolf. Like a druid, you can choose more powerful animals as you increase in level. These alternative animal cohorts work like the alternative animal companions available to a druid, but they are available as cohorts later than they are available as animal companions. When selecting an alternative animal cohort, use the list of alternative animal companions on page 36 of the Player’s Handbook, but treat yourself as a druid three levels lower than your character level....
Thus the wild cohort is much like an animal companion that, likewise, is acquired without templates. Further, the wild cohort's number of Hit Dice isn't specified; instead, the creature with the feat Wild Cohort has an ersatz effective druid level, which doesn't translate easily or directly into the wild cohort's Hit Dice or Challenge Rating. Finally, an animal is specified throughout the feat's description; a template that changes the wild cohort into something other than an animal renders the cohort invalid.
...But, hey, it's a game
If everyone agrees that instead of a regular ol' badger that a half-celestial half-dragon badger is somehow acceptable (like, maybe the guy with the feat Wild Cohort is a monk or something), that's great, and no one can stop you. Have fun.