Rexons are Plesiosaur-looking Badniks whose armored carapace can withstand the immense heat of their domain. Their backside (or shell) is indestructible. Their plating is light purple and light grey.
Sonic the Hedgehog 2
In their first appearance in Sonic the Hedgehog 2, Rexons only appear in the lava pools of Hill Top Zone. When the player gets near, the Rexons' head and neck will pop out of the lava and start shooting energy projectiles at the player from one position while their necks stay in motion. However, since the Rexons do not change their heads' direction, it is easy to defeat them from behind using the Spin Jump.
Since their body is invulnerable, the player must attack the Rexons' head. When successfully struck, the Rexons' head and neck will collapse while their main body is left behind to float in the lava pool. Since the playable characters will not take damage from touching the Rexons' main body, the player can use it as a stepping stone in the lava pools for purposes like getting closer to the Rexon's head to beat it.
In Sonic Blast, Rexon-looking Badniks appear in Red Volcano Zone Act 1. These Badniks are red instead of purple and do not appear that often. Similar to the Rexons in Sonic the Hedgehog 2, these Badniks emerge from lava pools and will shoot projectiles at the player when they get close, while steadily moving around in only one position. However, only their heads appear while their main body seemingly remains beneath the lava.
Like the Rexons, these Badniks do not charge the direction of their heads after coming out of the lava, making the easy to defeat. Also, like in every Game Gear/Master System game, these Badniks do not have Animals inside them.
In gameplay, the Rexons behave just like in their debut game: when the player gets close to lava, they will pop out and shoot at them, and only their heads can get destroyed while their bodies act as plaforms. 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. Also, upon destroying a Rexon, it will release an Animal.
Internal heat mechanisms allow the Rexons to spew dangerous fireballs from their mouths periodically. They are also equipped with heat-resistant protection that allows them to reside in lava without getting burned.
In other media
Sonic the Hedgehog Yearbook 1992
The Rexon made a small appearance in Sonic the Hedgehog Yearbook 1992' story "Space-Hedgehog" where it and a Spiker ambushed Sonic the Hedgehog, with Rexon shooting fireballs at him. After beating Robotnik, Sonic defeated both the Rexon and Spiker easily and released the animals inside them.
The Rexons' fins are never seen in gameplay due to their main body residing below the surface of lava pools where they are not visible.
There is a chance that the game may crash when destroying a Rexon's head. This is caused by conflicts in the code that causes the game to jump to a section of code earlier than it was meant to jump to, resulting in invalid opcodes being processed and an eventual crash. This would be fixed in REV02 of the game, included in Sonic Classics, though it is possible to fix this by modifying the game's code