So I've got some pets I need to get rid of. I'll release them if I have to, but some of them seem like a bit of a waste to just release. What are some general rules of thumb when it comes to pricing? And where would more exotic critters like a Mountain Guardian Lupora fall in that criteria?

It realy depend of the amount of visible traits, the amount and what they are, lucky rune and leaf vein usualy are less interessing then visible ones. For wild themmed, pets with more then 4 tp and up that were not mutated cost alot more then 2 tp pets, 1 vis are usualy a more expencive then 2 carried.

From what i can see for Mountain Guardian Lupora so far, most of them are quite expencive (at 200k) but a few are in the lower range (25k and less), your pup isn't tested so i can't give you an estimate but keep in mind that he have at least 2 unnown carried, maby more.

I tend to use this system, though it errs on the low side and doesnt include anything on themeds:
https://www.sylestia.com/forums/?thread=68895

While this could be a good base for rendom offspring and non themmed caugh pets you dont perticularly care about, this base is way too low to be used for anything else. It dosnt cover the food cost for the pet so you would basicly lose gold (and space) by selling anything that low as well as increass the risk of it being releassed by the buyer. From my point of vew any pets that are under 1k (not part of a givaway obviously) are better in the wild but i know some people dont like having their former pets releass, this is why i tend to warn others about that eventuality with low pricing.