Photo and brief summary on Boing Boing is here.
For the record, this really should be called something like a unicantler, since the structure growing from the fused pedicles is antler and not horn, but why quibble.
Boing Boing links to this article at nationalgeographic.com
It seems that the critter is a deformed roe dear, killed in Slovenia.
Coming soon: Air Force downs flying tea cup.