Volcanoids are game-exclusive monsters. They have lava surging through their veins. They can blow lava out of their head, and mouth. They have 2 legs, a large Eye of X.A.N.A. on their foreheads, and a turtle-like shell.
The first encounter with one is in Code Lyoko: Quest for Infinity. It's a boss fight in the Ice Replika. The Lyoko Warriors have never seen the creature before and Jeremie says that he will find his weakpoint. When the Lyoko Warriors start to fight against the Volcanoid, they can't damage it because of its lava head. Because of that, Jeremie says that he found the weakpoint. He says that Ulrich's Triplicate is the most effective against it and takes the lava from the head temporarily away and then, the Lyoko Warriors can damage it. However, they should do it fast or else the lava will return to its head and Ulrich needs to wait until he has enough power to use Triplicate again.
Later, they become a common hassle in the Volcano Replika.