One of the first abominations to come from Phyrexia, a truly amazing creature. It started out as human, but the processes inside Phyrexia changed it into a fighing machine. The creature's body somewhat resembles a giant sloth, but it moves like a horse. Its mouth is wide, and its jaw can unhinge to allow it to open almost 180 degrees. Fully open, its mouth can accept a whole human head, which its sharp teeth and strong jaw can then remove from the body. The steeplejacks were cunning, but it was a brutish form of cunning in which everything was battle and battle was delightful.
The Priests of the Fane of Flesh oversaw the making of these beasts, but they were designed by Yawgmoth himself. The most famous of the steeplejacks was Xod, Yawgmoth's first disciple.
information taken from The Thran by Jeff Grubb