I believe you mean a horse's NECK, since the mane is made of HAIR.
Horse Fat (I'm not kidding)
On this week's Iron Chef they used an ingredient they said was the fat at the base of a horse's mane. They used it raw! It's considered a delicacy, as I understood it. -Chowhound 10/1/2000 (original episode date)
Horse meat sashimi. Fat is white pieces. I've eaten monkey meat on a stick with bbq sauce. Tastes like chicken!
Maybe it had fat hair?