non-class or feat animal companions

[Yogibear41]

If a character who is neither a druid or a ranger(or a similar class) and who doesn't have the wild cohort feat, has a pet riding dog or something similar to it does the critter ever gain experience or does it simply stay a 2 HD creature(in the case of the dog) forever?

[Hurnn]

Stays the same forever, A&EG has better animals you can buy as you get higher.

[Thurbane]

You could apply the Warbeast template (MM2) by training the dog...

Handle Animal DC 20, takes 2 months.

[Hurnn]

good call.

[Yogibear41]

Okay thats what I thought, thanks guys.

[Coidzor]

The only way to get one is either to just buy one and use the handle animal stuff for it. And then from there you (may) apply the warbeast template depending upon whether you view it as an inherited template or something that can be acquired by training. I believe the text itself is closer to the first, IIRC, but it's a popular houserule if it is a houserule to allow it to be applied to extant creatures.

Other than Warbeast.... I believe the only way to add HD to it would be to Awaken it and make it into an NPC ally. Might be one or two other little tricks and loopholes though.

Well, there might be some byzantine way to get access to the old 3.0 animal friendship spell despite it not existing in 3.5, but even there that doesn't help make existing animals more potent.