Back in 2005 a software company called Lionhead Studios released a game called "The Movies." It was a machinima engine at it's heart, and since YouTube wasn't invented yet the studio hosted their own website for videos made with this game. One of the top 10 most popular videos ever produced was a surprisingly lengthy comedy sketch by a group of German players. Right from the start there was, for no apparent reason, a horse thrown into a scene. One of the narrators piped "Why is there a horse here?" and the other responded with, "Who cares? I'm a horse!" and later "Fuck logic, I'm a horse, I'll do whatever I want!"
In the comments this joke was repeated many, many times by people. The community quickly turned it into a running gag and the "horse joke" soon became a staple of many comedy routines made by many people who would insert it into their own videos at seemingly random and pointless junctures.
Eventually Lionhead studios went out of business, the website was taken down and all of it's content lost forever. The community, now disbanded, has long since spread the horse joke around to everything else they've come into contact with, and eventually it became popularized simply in itself. Today it is mostly used by people have no idea as to it's true origins.
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