The Sycorax horses had, like the Sycorax, both an endo- and exoskeleton, but instead of just covering thir heads, they covered most of the body, with bony tails and bone-like spikes over their hips.
When a second Sycorax invasion of Earth was tried by the widows of the Christmas invasion of 2006, a few Sycorax riders mounted then to patrol the coasts of Shadow Cay. Later the Tenth Doctor stole one of them to escape his persecuters. All the creatures were presumably killed when Shadow Cay exploded. (COMIC: The Widow's Curse)