Rabid Dog is an Emblem Heartless that appeared in Kingdom Hearts II in the Underworld. It has a blue dog-like appearance with the heartless symbol on his back part, and mostly relies on instincts during battle. A pack of them are seen attacking Meg in the Underworld, before getting scared off by Sora.
Rabid Dogs are some of the most harmless enemies. Most of the time, they will stand, and look at Sora, getting to sleep from time to time. However, sometimes they may bark, the bark working as a sound wave and causing damage. A relatively easy enemy that tends to appear in packs.
A variation of it can be found on the Cavern of Remembrance called Bark Hound. They, unlike the Rabid Dog, will attack you viciously with Blizzard magic. The Reaction Command, Quick Detention makes Sora swing it to the air and finish it off. You have to defeat it quickly before it disappears.