the.com/mudpuppy

A salamander that never grows up, keeping its frilly baby gills for life underwater.

means A large aquatic salamander (genus Necturus) native to North American lakes and streams that retains its feathery external gills throughout adulthood.

from An American English compound joining plain old 'mud' with 'puppy' — a name born of folk belief rather than biology. The popular story held that these slick, lurking creatures could bark like a dog, and old anglers swore they were poisonous (they aren't). The 'puppy' likely nods to their small, soft, almost cuddly look as they paddle through the muck. Their permanently juvenile gills also earn them the related nickname 'waterdog.'

eternal child
Stays in larval form forever, gills and all
red plumes
Bushy external gills wave like feathery antlers
squeaky myth
Named for an alleged dog-like bark it cannot make
big lungs too
Has lungs but barely uses them underwater
cold lover
Thrives in frigid, oxygen-rich, fast water
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