Little is detailed about the Headless Horseman's life before Dreadpool began murdering the inhabitants of the Ideaverse, however it can be presumed that it followed much the same path as Brom Bones from the novel The Legend of Sleepy Hollow.
Whilst travelling through Sleepy Hollow as the Headless Horseman, Brom Bones came across Dreadpool, who cut open his horse before beheading Brom, hollowing out his head and turning it into a Jack O'Lantern.
Later Headless Horseman and other characters from Ideaverse were resurrected by Sherlock Holmes, who has restored that universe.
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A black horse.