Mipsy

Mipsy is the nickname given to a female version of Mips the Rabbit, which can appear in certain personalized copies. She appears as an untextured and somewhat unfinished pink version of Mips, although some people claim to encounter a variant which has a finished model and textures. She always appears on the Third Floor, and usually appears only once after 60 stars have been collected, although may appear after more or less.

Behavior
As opposed to Mips's consistent speed, Mipsy's speed typically varies, usually running faster than Mips, although sometimes she is slower. Players who encounter Mipsy say they were frustrated by Mipsy's movement patterns, often making turns that are sharp and unpredictable. She typically runs up the stairs that lead to the entrance door for the Endless Stairs, and occasionally jumps down over the banisters, which players claim was extremely aggravating to deal with.

Speculation
The most common belief is that Mipsy was the beta version of Mips, evidenced by the pink rabbit found in the gigaleaks. Mipsy was most likely replaced by Mips due to the somewhat frustrating movement patterns.