From the Super Mario Wiki
“Yeeoww! Unhand me, brute! I'm late, so late, I must make haste! This shiny thing? Mine! It's mine. Finders, keepers, losers... Late, late, late... Ouch! Take it then! A gift from Bowser, it was. Now let me be! I have a date! I cannot be late for tea!”
MIPS the rabbit is found in the basement of Princess Peach's Castle when the player acquires fifteen Power Stars during the events of Super Mario 64. When Mario approaches MIPS, he runs away. After Mario catches MIPS, the rabbit gives him a Power Star. MIPS appears in the basement a second time after Mario has collected fifty Power Stars. Mario can catch MIPS again and will get another Power Star from him. In the remake Super Mario 64 DS, there are many different rabbits modeled after MIPS, but they do not give up Power Stars. Instead, they give up keys to unlock minigames in the Rec Room. Two of Mario's and one of Yoshi's rabbits can be found in the same location MIPS was in the original game. Along with Mario, MIPS was the first character created for Super Mario 64. MIPS was used extensively in early Nintendo 64 test simulations and ultimately made it into the final game because the development team liked him so much.