So they have a medium to short length tail full of jelly-like goop, and is actually more vital to them than blood. The skin of the tail is a thick rubbery film. The gloop helps regulate and store vital minerals, vitamins and mysterious gooey substances that keep them fit and healthy.

They're pretty bendy as long as the tail isn't overfull, though the thickness of the skin and the contents of it means they could get injured if the bending angle is too acute.

If the gloop keeps building up in their tails they're at risk of uncomfortable swelling and maybe even a rupture. But critters have natural sphincter-like pores across their tails that gradually release excess and old goo from the tail. This is why a health critter often has a damp, slick tail with goo dripping off.

If they are low on goo in their tail, they become frail and reclusive until they can't build it back up again. If attacked by a very formidable for that manages to amputate the tail at any point, the Critter runs the risk of 'bleeding out' and it is almost always fatal.

For this reason Critters are a mostly feisty wild species, having developed massive chunky claws for digging, climbing and fighting. Their skulls are developed to deliver a fierce bite. Though the largest ones are only 2 and a half feet tall, Critters live in large swarms and take safety in numbers.

ORIGINAL HEADCANONS