I haven't run into the following, as far as I can recall:
- tardigrades
- coconut crab
- sea cucumber
I have a feeling that trying to write a fantasy story based on any of these would come out decently weird, if you're up for a challenge.
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I haven't run into the following, as far as I can recall:
I have a feeling that trying to write a fantasy story based on any of these would come out decently weird, if you're up for a challenge.
sea cucumbers
Did you mean: Elder Things?

A Pathfinder podcast i listen to (BCBParty) does have a sea cucumber character, but he's a familiar so i'm not sure that counts.
Axolotl, groundhog, sea urchin.
While you are correct in pointing out that they're underrepresented, by shear coincidence two of the games I've been playing recently feature an urchin and axolotl. There's a sea urchin background character in Mina the Hollower, and one of the playable characters in Windblown is an axolotl.
The red panda. Aside from Turning Red and, oh yeah, Mozilla Firefox, they don’t get much love in popular culture. They don’t even have them in Animal Crossing!
There’s the Ponya from Torchlight 2
edit: also re: firefox, AFAIK they've been using a red fox as a mascot at least since 2006 when I first discovered it, at least they already had the classic fox and globe logo.
giant squid
Cthulhu: "What am I, chopped liver?"