Very very nice. The design of this monster is very easy to grasp from first glance. The main area for improvement however is the dimensional perspective; the heads all seem twisted and contorted towards the camera, with their horns slanted to one size - breaking symmetry with the face and especially the jaws of the heads.
For a thumbnail this is excellent, but as a concept art illustration the perspective would need to be rectified. A simple piece of advice would be to sketch out the 3D shapes of the Hydra(?) using cubes and prisms to plot out your perspective, this would keep things looking symmetrical and alluding the third dimension.