Based on the regions we will eventually have available, I think I came up with a few ideas.


Flame region
Flame Salamander/Lava Monster - Much like the salamander mentioned before, but with added flame feature. Possibly a bulbous flame sac? I slashed it with lava monster to give a distinction/idea of what I was thinking of (part salamander, but partially made from lava).

Erupting Rock Tortoise - A tortoise based creature whose shell is made of hardened sulfur and lava rock with the potential to erupt with volcanic activity.

Mountain region
Gonna second the idea for a ram based pet.

Gryphon - Only unlike the traditional hawk/cat hybrid, thinking of an owl/cat hybrid.

Jackalope - 'nuff said

Manticore - This could easily be moved into either the flame or desert regions as well

Snow region
Yeti: Yep. Maybe toss in some crab?

Desert region
Gonna second the scorpion idea here.

Hyena - Seems appropriate?

Rainforest
Hydra - Marshy, evil creatures of multi-headed awesome

Chimaera/Chimera - Amazing mixture of snake, dragon, lion, and goat

How about a bat

YES!

And that bat, can "Mutate" and be on
FIRE! >:^O
Sorry, I just find Fire-Bats funny~ :3
Could have Mutation on Fur around neck
and stuff~ <333

Mammals are waaaaaaaaay overrepresented so far.
Consider this: mammals represent 0.003% of all species.^ That is ridonculous! Furthermore, every current pet is a vertebrate. Yet the vast majority of species are invertebrates: about 97%.

There's a lot of potential in some of the invertebro posse, escpecially underwater. Giant clams, octopuses and jellies for starters. Shadowphyr posted an epic cute pic of a nudibranch on the previous page that is just irresitible. I seriously love that thing. And who doesn't love cuttlefish or nautiluses?
Insects are the largest group of animals in the world, from which there are tons of cool critters to choose like moths, praying mantises, bees, fireflies, stag beetles, butterflies... the list goes on forever. Though not an insect, a scorpion would be cool for the desert region, as mentioned by others ( I would love to see a cactus critter there btw, just saying ).

Even other vertebrates that are not mammals would be cool. Fish, amphibians, reptiles and birds are huge categories on their own. And I still have a serious plant fixation, even mushroom critters would be rock-awesome. Really, there are all kinds of awesome non-mammals to choose from. Not hating on mammals here, don't get me wrong --- I just think there's a huge amount of diversity that can be found from looking at other species groups.

I'm glad everyone likes the bat


as for the mammal problem
how about anomalocaris canadensis
here is a picture so you know what it is

Momo is from avatar the last airbender, unless there is actually an animal called momo
he's a flying lemur

also: how about a leviathan?

How about red pandas' and raccoons? that would be sweet

Wolves or eagles would be awesome.

You could add something like the Dinosaurs, as in the Brontosaurus. Or you could do something like the monsters from Greek myths like the Hydra or a Chimera.