The Rexon is modeled after a Plesiosaur; it has a round, armoured body with four fins that allow it to wade through bodies of liquid and its head is connected to the body by a segmented neck consisting of four rounded segments. Its head and back are normally the only visible parts as the rest of it is usually submerged.
The Rexon has also been comically dubbed the "Lavasaurus" due to its ability to swim through lava. Its body is designed to withstand intense heat and it can also shoot balls of fire from its mouth.
Sonic the Hedgehog 2
The Rexon makes its first appearance in Sonic the Hedgehog 2 on the Sega Mega Drive. Appearing in the Hill Top Zone, it sits in pools of lava waiting for the player to come close. When the player is within range, the Rexon's head and neck will emerge and start periodically spitting thermal projectiles. A jump attack to the Rexon's head will defeat it, though the dome of its body will remain after its head and neck are destroyed. The Rexon's body can then be used as a foothold. It should also be noted that the Rexon does not turn its head and so will only shoot in one direction.
The Rexon reappears in the Red Volcano Zone of Sonic Blast on the Sega Game Gear. This variant is coloured red rather than purple and when they emerge from lava pools, only the head and neck appears, so the body cannot be used as a platform. As with the previous variant, Rexon attacks by shooting flaming projectiles at the player and can only fire in one direction.
Rexons return once more in Sonic Mania's Lava Reef Zone, retaining their appearance and attack pattern from Sonic the Hedgehog 2. This time however, the player can ride one of the walker mechs in the Zone over a Rexon to destroy it completely, body and all. Doing this earns the player the "Without a Trace" achievement/trophy.
Sonic the Comic
The Rexon has made two notable appearances in Sonic the Comic. In the Issue #20 story "Hill Top Terror", a Rexon was encountered by Sonic and Tails in the Mountain of Destiny, guarding Dr. Robotnik's Seismotron machine. After almost being doused in lava, Sonic became stressed to the point where he transformed into the maniacal Super Sonic. Super Sonic destroyed the Rexon by flying down its mouth and ripping it apart from the inside out.
In the Issue #59 Sonic's World story "The Seven Badniks, Part 1" Sonic and Tails fought against a Rexon once more in the Hill Top Zone. They weakened it by spraying it with fire extinguishers, cooling it down enough for Sonic to destroy it with a Spin Attack.