The Hydra is part of the final boss fight of Skylanders-Spyros Adventure.
The Hydra has 4 heads that look vagely similar. The Blue head looks like a smooth eel with a blade on it's head. The Black head resembles a Urugaan from Monster hunter. It 4 tubular things that are that are covered in orange colored lava. Cracks filled with the same lava are visible. The Green head looks like a Dragon head with 4 horns. The Undead head looks like a skeleton of a Dragon with yellow eyes and two horns.
The Blue head summons ghostly "Doom Sharks" that damage you if they hit you, but heal you if you dodge them. The Black head causes volcanic orbs and cracks that damage you. The Green head will either summon green orbs and snails that follow you (both hurt you), or it forms a half ring around Kaos. If you attack the other side of Kaos, A ring forms there too. The Undead head summons geomectric patterns that will light up and transform into lasers that damage you.