The Centaur effect is a discontinued effect sometimes referred to as the Horse effect. It is found in Pholus' studio in The Marble Hall by interacting with whatever is under the tarp. The effect gives Usotsuki four feet and horse ears.
As of version 0.03, the effect has been replaced with the Chimera effect and can no longer be found in Pholus' studio. Instead, it is found in a section of the Sewers that can be accessed with the Snow Bug effect. The new effect is visually similar, and pressing Shift will make Usotsuki meow.
To access the replacement, go through the Grandfather Clock in the Bird's Eye Garden and use the Snow Bug effect to travel through the Sewers until you reach a strange NPC shown on the left who will give you the effect.