A koopa, "inspired" by the Glitz Pit enemy in Paper Mario: The Thousand-Year Door. It'll turn to a rainbow invincible shell when stomped, and will ram towards the player if flipped with a cape.
Well, it's a well thought sprite, but I'll have to reject it again, for the following reasons:
-> When flipped by a cape swipe, the upside-down disco shell does not bounce off walls;
-> When flipped by a cape swipe, the upside-down disco shell does not interact with Yoshi and other sprites at all;
-> Minor: the death animation is not what I'd expect from a dying Koopa (just the shell falling off). Not a removal reason; the same could be said by the wonky Y speed that happen sometimes when you kill the sprite with a star;
-> The disco shell can be killed by fireballs, whereas a spawned shell-less Koopa cannot;
-> Mario cannot even spin-jump the flipped disco shell without hurting himself;
-> Sometimes, spin jumping over the sprite when wearing a cape can make it disappear, but the shell-less Koopa still spawns.
That's all I've found. Further questions may be answered by DM.