Chaotic mobs spawn deep underground, in areas where there is very little light. They have very little defense, but very high attack. They would be very common on the "dark" worlds that are being implemented. One major attribute to chaotic mobs is the fact that they cannot SEE players, but instead are attracted to heat and sound. This means if your character stands still, the mobs would not aggro. BUT if your character is moving around or has a torch or heated item out, they will. As a result, these mobs have a significantly higher aggro range. Visually, the monsters have little eyes and are generally repulsive looking, think deep sea creatures.
I like this. Some darkness-spawned creatures will bring up all those childish fears sleeping in us. And eventually make game more interesting and deep. I think these mobs shall be really rare overall so that they will entertain the player rather than bore him. Here is a picture my imagination brought up.