Larger version of the good ol' centipede big snakey type mob (new model and animation) - poisons players on attack.
More health and attack damage as it's size is larger.
Drops stuff?

Millipedes are generally more safe to handle than centipedes in real life - perhaps they should be a passive mob?
This could even potentially be a first in Minecraft; a creature with many mounts, allowing 5 or 6 players to ride it at a time!