A Regular Mook is the standard opponent and the most common. Toting an assault rifle, he'll lay down fire when he detects a bro until one of them dies. He can be parachuted and spawn from a Mook Door. Despite their low rate of fire, they are one of the more common source of deaths for anyone who does not pay attention to the "!" symbol.
The Suicide Mook will search for bros, charge them and detonate. This can be a game changer in tight quarters. You're best to knock these mooks back from a distance. They will detonate shortly after they are dead. He can be parachuted and spawn from a Mook Door.
A Riot Shield Mook hides behind his riot shield (that can withstands some bullets but will lose it after a melee attack. His riot shield cannot stop head crabs though.) but he is unable to fight back. He can be very help full to stop bullets from hitting other Mooks alike.
A Big Mook can take more damage and spool up a minigun.
A Scout Mook looks for bros and after detecting one, runs around alerting other hostile units of their presence.
An attack dog is a close range opponent that will jump into nearby bros. It will feed on a corpse when it finds one until its destruction and will then howl before becoming a bigger and stronger version called Super-Size-Me Dog. The attack dog is drawn to the sound of gunfire, and will automatically detect a bro if it hears a shot.
An Armored Mook will resist to a tremendous amount of bullet damage and will shoot very dangerous flurries of bullets, but he is weak to falls. Bros can jump on him. After his death, bros can ride in its armor (Entered using the melee button): his fuel will deplete from jumping, and he can fly (something the mook never does). Fuel can be replenished with ammo crates. The Armored Mook can also be self-destructed (using the special attack button) to do moderate damage to the terrain and surrounding mooks. Will explode when it has taken too much damage from falling and explosions. Being hit simultaneously by two Armored Mooks' fire will cause your bro to jump out (and likely die).
A Grenadier Mook will toss grenades in a bro's direction, and also drop one when they die. Each time one of their corpses is hit, a grenade is dropped from it. His grenades can be thrown back by pressing the melee attack near them.
A Bazooka Mook will launch destroyable rockets to bros. Their rockets are heat seeking, making them very dangerous.
A Ninja Mook is an opponent who lays down until a bro gets close enough. It can be hidden by a Foreground Bush Doodad Or Alien Fronds but a tiny bit of the Ninja Mook but a pixel can still be seen.
A Jetpack Mook is an opponent who hover if seeing a bro. Upon death its corpse acts as a Propane Tank going to the direction he's facing before exploding. They have the same fire rate of a Mook.