Rat

Rats are creatures that mainly appear in the Wario Land series, namely in Wario Land: Super Mario Land 3 and Wario Land 2. They have rodent-like traits but their appearance and behaviour differs between the two games. However, they share the fact that they oppose Wario in some way.

Role in Wario land: Super Mario Land 3
Rats make their first appearance in Course No. 5 in Rice Beach of Kitchen Island. They resemble rodents, but are humanoid. They can be seen wearing bandannas so it can be assumed that they are members of the Brown Sugar Pirates. They wield throwing knives for battle, which they will toss at Wario when he gets in range. When Wario gets hit, he will regress to Small Wario, or lose a life. When the knife hits a wall, it will stick there for a moment and then disappear. Wario can jump on it then.

Role in Wario Land 2
Rats reappear in the sequel, but behave very different and also show a completely different appearance. They look more like actual rats, swowing a red colour and are also noteably smaller than in the previous game. They emerge from mouseholes and attack Wario by tackling him. The attack is too weak to do any damage and Wario will only bounce back a few steps, but in certain areas, this can be a hindrance when they bounce Wario off of a small platform. It is noteably that rats cannot be stunned by Wario's ramming attack and thus cannot be picked up and carried around. When Wario defeats a rat, a new one will respawn after the player re-approaches the mousehole.

Rats bear a slight resemblance to Little Mousers in Wario Land 2

Trivia
In Paper Mario: The Thousand-Year Door, Jolene told Grubba that the disturbance in the Glitz Pit storeroom must have been due to a "rat or something" (which was probably a pun on Ms. Mowz' presence in that room). This explanation calmed Grubba, implying that a rat, or at least certain rats, are common pests in the Mushroom World.