The anti-slow bloon is a parent of the pink bloon & a son of the Ark bloon. It can not be slowed down via non-stunning methods, but can be knocked back. It can be identified due to the liquid inside the mysterious crack that it has. When popped, this liquid spills out onto the track & can make 10 more bloons immune to slowdown effects that don't stun.
The Keener Bloonopedia reveals that the liquid inside of it is what prevents it from being slowed down.