Rally biter - acts as a scout. The first one travels from it's nest towards your base in a straight line until it gets killed, the next rally that spawns knows where the previous one died and travels along a slightly different angle to the prior one. Eventually one rally will make it to a point near your base but out of the range of your defenses. It then calls all of its buddies over to attack from this new spot.
Acidic biter - Wall killer. Gets close to your walls then detonates. Contains concrete dissolving chemicals that when triggered cause massive damage to any walls within a 3 tile radius. Resistant to lasers but weak to guns. If killed before it's able to trigger the chemicals the damage is greatly reduced.
Silver biter - strongly resistant to lasers.
Stone biter - strongly resistant to guns.
Reaver biter - Acts like the Reaver from Starcraft, spitting out small exploding biters from long range.
Electric biter - can cause one, or more, of your turrets to turn on its neighbors.
Please post your ideas.