From Sonic Retro
It's a chameleon-like badnik apparently based by the Newtron, both in design and purpose.
The Leon made its debut in Sonic Adventure, where it has the ability to turn invisible to avoid detection and sneak on enemies with its tongue and tail lashing.
|Sonic||Windy Valley, Lost World|
|E-102||Windy Valley, Hot Shelter|
The Leon made a second appearance in Sonic Advance, with a complete new design and attack. It crawls around back and forth, and his method of attack is by sticking its large tongue. It shows up in Angel Island Zone.