Usage online

"I own a horse" is a phrase said to troll people on chat websites, for example Omegle or ChatRoulette. When someone says "I own a horse", the partner is said to disconnect.
It's also sometimes used to fend off sexual predators, and people looking to cyber.

Origin

The exact origin of the phrase "I own a horse", such as who was the first to say it and when, is currently unknown. Research into this has proved difficult, as the meme generally is used on chat sites which are both anonymous and typically do not keep public archives of user's conversations. However, there have been several internet memes and viral videos that are similar in content, and may have acted as precursors to this meme.

In this song by the Rubberbandits, posted on Youtube in 2010, they talk about how they "Have a horse outside", basically telling the other white trash in the video "Fuck your car, I own a horse". This is the first instance known where horse ownership is used to put others down, and is much closer in usage to the "I own a Horse" meme as the ownership of a horse is used to shut down others solicitations. Whether this is directly related to the earliest instances of the catchphrase is unknown.

The meme in action generally looks something like this: