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.

thanx for advice, i will keep that in mind for the next one....
i also thought about lighten up some of its parts that are farther away from the viewer...
it was done from silhouette

some secret knowledge for u all...
it is living in the mountains and can float thru manipulation gravity. Each head has its own attribute. His right one spits toxic gas, left one acid and the middle one fire, making it a troublesome enemy. Many hunter searched for it... none of them returned....

Yeah, this looks awesome. You pushed the back of the Hydra away from the heads, making it easier to differentiate between head and tail. Bravo.

In addition, as an onlooker I do not know the desired vision of the artist. And in many cases symmetry is not the desired outcome of the creator, asymmetric monsters look more villainous and chaotic for example and can give them a more weathered appearance.