This critter is encountered in the cave in 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.
"Rabid Dog" is Rex's more powerful form, which spawns if the cave remains in darkness. A Spiked Mut (a weaker variation) will be fought if a Match is lit upon entering the cave.