The tail is part of Wobbuffet's body.
Wobbuffet is a tall, cyan-colored blob-like creature with a black tail that has two eye-spots on it.
In theory, however, Wobbuffet's body seems to be some type of protection for its tail, as implied here:
Wobbuffet seems to be highly skilled at countering enemy attacks, but not much else. It is also greatly tolerant of enemy attacks and can endure more attacks than most other Pokémon. If it is knocked out, it is capable of taking the attacker down with it, although it will usually only attempt this if its tail is attacked.