MIPS is a rabbit found in the basement of Peach's Castle during the events of Super Mario 64. When Mario approaches the bunny, MIPS will run away. After Mario catches MIPS, the rabbit gives him a Power Star. MIPS appears in the basement a second time after Mario defeats Bowser. There are different colored MIPS in the sequal Super Mario 64 DS
Apparently, MIPS is late for an important appointment. This is a pun off of the bunny from the cartoon and book Alice in wonderland.
MIPS gets his name from the measure of a computer's processor speed, which is measured in Millions of Instructions Per Second.