The electric pony is a purported sex act in which a dominant man ejaculates over the back of a submissive partner, then covers their semen with pubic hair or hair from around the anus to create a “mane.” Once the mane is in place, the submissive person becomes the pony. They may then act like a pony, either because they are forced to by their dominant partner or because they enjoy pony play.
This term appeared in Urban Dictionary in 2006, but there are few other references to it online, even in porn. It may be more of an online urban legend than a practiced sex act.