This trend was started by an art of "Shonbori Rudolf" (Feeling-Down Rudolf) drawn by Deon (post #5635540). It eventually turned into something like a microcosm of Umamusume, Horse Racing in general and general Internet memery, courtesy of the various anons at Futaba Channel. A lot of the content consists of short videos with limited animation, usually with the character flailing their limbs on their back (the Jitabata or "flailing" animation) or, more frequently, imitating the Komaru dance.
Much of the content depict the horse girls with exaggerated personalities and traits, or warped personalities (i.e. Special Week turning into a deadbeat father with gambling issues, squandering her daughter BuenaVista's earnings in failed bets; anything involving Zakkoku...). They aren't limited to the official characters, unmade or not and those original characters can be surprisingly well fleshed out for what should be glorified shitposts. In time, those original characters eventually outnumbered the official characters by a large margin.
This tag should be used whenever the artstyle is used or reproduced (character is drawn like Shombori Rudolf, character does the Jitabata animation or the Umaru Dance...), whenever it refers to a particular video or when a unique character design for a original character from those videos is used.
