Ahuizotl

That must be a Ahuizotl. 

A fearsome legendary Creature is described as dog-like, 

its waterproof fur often clumping up to create spikes. 

Ahuizotl has a Hand on its Tail and it's from Aztec Mythology. 

Ahuizotl lived in or near the Water of Mexico and it used its Hand

on the End of its Tail to snatch its prey, 

dragging the person into the depths to drown them. 

This is an Ahuizotl in "My little Pony: Friendship is magic" 

appeared in the Jungle of Mexico. 

He has a normal skinny Belly, 4 Legs and a Hand at the End of his Tail.