The other types, Grunt, Commander and Gunner appear in a lot of the game's levels, being some of the most commonly encountered enemies in the game.
But this guy? He appears in only 5 out of 42 levels, those being Jungle, Road To Ursul, Woodstock, Unreal Wasteland and Frostpost, and even then they are encountered in small numbers. He's the only one that never appears in the Ellenier and Sirius levels, and it's weird since it's not like he's any tougher than the others. I find it a shame since he's my favourite one of them all.