Hellhounds were creatures that had features of dog (and/or wolf), saber-tooth tiger, possibly bear, and a few other things. They are about six feet long and three feet tall, with a dog-like body and a cat's head with long saber-like canines. They have overdeveloped shoulder muscles (similar to a bear's), clawed feet with rear-facing spurs, and twin barbed tails.
The companions encountered a pair of these creatures in the Marshall Islands (Savage Armada) that looked exactly like normal mountain lions, but have two tails. Given that there are several other bio-engineered species on the islands, it's likely that these are a variant form of hellhound, not muties.