This critter is encountered at the Cave near Highpool. Its true identity is "Rex", Bobby's dog, which has somehow become feral due to disease. Bobby hides Rex in the cave to recover, fearing that it will be put down otherwise. If slain, Bobby digs a grave for it, and will not forgive the Rangers for their allegedly heinous crime.
"Spiked Mut" is Rex's weaker form, which spawns if a match is lit upon entering the cave. A Rabid Dog (a more powerful variation) will be fought otherwise.