It's a chameleon-like badnik apparently based by the Newtron, both in design and purpose.
The Leon, also known by its model number E-04 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.