You see a strange rat
Notes
They sometimes spawn on characters while inside the Cobra Bastion if you have the Cobra's curse. To remove the curse you have to go to the +3 floor and toss some gold on the floor there.
Abilities
Melee (?)Damage Taken From Elements
Physical 
Death 
Holy 
Ice
Fire
Energy
Earth
Location
Behaviour
Unknown.
Strategy
Just kill them with any attack.