Mouser

Mousers are large, humanoid mice who wear sunglasses, gloves and boots; they are a resident species of Subcon, the land of dreams. Not much is revealed about Mousers, though it is mentioned that they are somewhat vain and utilize their bombs to destroy good dreams.

Super Mario Bros. 2
In Super Mario Bros. 2, two Mousers appear as bosses, one in World 1-3 and the other in World 3-3. In battle, Mousers throw bombs at the player, which will explode after a short amount of time. In order to injure a Mouser, players need to grab one of these bombs and throw them back at the Mouser so that it explodes near the creature; doing this several times will defeat them. Both Mousers are slightly different in color, and the second takes more hits than the first. In Super Mario Advance, a new boss, Robirdo, took his place in the third world, but Mouser replaced Triclyde as the boss of World 6.

The Super Mario Bros. Super Show!
In The Super Mario Bros. Super Show!, a single Mouser, only ever identified by the name Mouser, appears as a member of the Koopa Pack and as the second-in-command of King Koopa.

Nintendo Comics System
In Nintendo Comics System, Mousers make some appearances, though in the comics they are renamed Mice, with the name Mouser being applied to a single member of the species.

"A Mouser in the Houser"
In the Nintendo Comics System issue "A Mouser in the Houser", it is revealed that the Mice once had a prosperous society, having dug the Transport Tube system and hidden all the treasures they found in it. Eventually, the society of the Mice began to crumble when their king, Mouser, abandoned them and joined the forces of King Bowser Koopa.

In the aforementioned comic "A Mouser in the Houser", several Mice abduct Princess Toadstool after she becomes lost in the Transport Tube system. After kidnapping the princess, the Mice reveal their origin to her and offer her the position of their queen, saying that if she agreed to this proposal, she could have a map of the Transport Tube system, as well as dozens of ready troops to help her defeat Bowser. Princess Toadstool eventually rejects the offer of becoming queen of the Mice when they refuse to help her aid Mario, who was being attacked by Mouser.

"The Buddy System"
In a later issue of Nintendo Comics System entitled "The Buddy System", a large group of Mice appear to capture Mario and Bowser when the two become lost in the Transport Tube system. Shackling Mario and Bowser together, these Mice interrogate them and learn that Bowser had planned on flooding the Transport Tube system by blowing up the main water valve of it. Deciding to aid Bowser in his plot of destroying the Transport Tube system, this group of Mice use bombs to blow up the main water valve, which causes a large amount of water to flow through the Transport Tube system and wash them away.

"Bedtime for Drain-Head"
Though several Mice appear on the alternate cover of another Nintendo Comics System comic entitled "Bedtime for Drain-Head", no Mice actually appear in the story.

Super Smash Bros. Brawl
Mouser makes a cameo as a Sticker in Super Smash Bros. Brawl.

Trivia

 * In Mario Party Advance, a Squeak named Mouser appears; despite being called Mouser, Mouser has only a minor resemblance to an actual Mouser.
 * In Yume Kojo: Doki Doki Panic, a third, white Mouser is fought as the boss of 5-3. When the game was converted into Super Mario Bros. 2 for North American audiences though, this white Mouser was taken out of the game and replaced by Clawgrip, possibly because it was perceived as being too powerful.
 * In Mario & Luigi: Bowser's Inside Story, after Bowser gets to his old castle which was taken over by Fawful and renamed Fawful's Theater, a Shy Guy says "This castle... it had a name... long ago... Browser's Castle? Mouser's Castle? Wowser?"
 * Mouser could be a pun on Mauser, a German gun company.