I've heard of this before, too, along with the same sort of hair along the midline of the belly. I don't think it's TOO terribly unusual, and does not indicate that anything is wrong. I think it's most often seen in hairy, hardy type horses.
I've heard that this is a "primitive feature" like dorsal stripes and the such only less common. I've seen it in zebras before and heard of some horses presenting with it all the way down their backs!!!
Clancy 17hh chestnut Dutch WB, '99. Owned and loved since '04 and still goin'!
I don't think this is the first year the pony has gotten the hair but I am unsure how long they have owned him. Somehow it almost reminds me of a pilonidal cyst. Only it didn't cause a cyst. Just the extra hair follicles just grew!
Never saw THAT before, but my guess would be a few tail hair follicles grew in the wrong place in-utero. Have some fun with it--make him a little funky topknot or something--would totally freak people out!
Weirdest thing I ever saw was a school pony we used to have, who had sustained a coronet injury at some point resulting in his constant growing of a big bony "toe," like something prehistoric, straight out above his hoof. THAT used to freak people out, too--and he wasn't fond of me taking it off with the nippers.