Applying the dire template to an adult animal

[danzibr]

First, let me say I know there's no dire template in official books. I plan on using something like this (http://www.dandwiki.com/wiki/Dire_%283.5e_Template%29) sans the extra HD (or at least not double).

With that said, here's my question. I'm doing a one-person campaign (I'm the DM). My wife, the player, has a Cheopard (Cheetah + Leopard). I don't want the Cheopard to start out huge, but get stronger over time. At some point I want her Cheopard to pick up the dire template. My question is... how?

Should it be some sort of magic? Or perhaps the Cheopard goes into something and ages (so was dire all along, just young). Or... well, those are the two things I've thought of.

Also, rather than starting a new thread, another question. Does War Trained (I think that's the template name) HD stack with Wild Cohort or AC HD? I think not, but just want to double check.

[Coidzor]

Warbeast from MM2? Should if you allow the template to be applied at all, yes. Animal Companion HD are bonus HD, the creature would lose them if it stopped being an animal companion(at least, probably), whereas Warbeast just gives the creature 2 more HD in the same way that Awakening the creature would.

You could have some sort of ritual be done, or you could just slip it in there somewhere as part of the natural animal companion progression as the character levels up.