On one of my dexxers, I run my pure black horse because I've forgotten how to drive a swampy and seem to get him stuck on the edges of rocks, plants, trees, etc. too often while running at full speed.
Most of my characters run beetles (smith/tailor), ethy Llamas/beetles/unicorns, or chargers of fallen. When going into a PvP possible situation, you have to have a REAL pet so when you get dismounted you can hit your "all follow me"/remount UOAssist macro and get out of dodge, hopefully before you see gray and are OOOOoooOOO-ing.
Since you are not able to get them anymore, the "oddity" factor is going to raise the prices on them. I obtained my own and a few others for people a couple years back. Back then, they went for ~2mil. I think due to not being able to obtain them anymore, they are somewhere in the 5-8 mil area and that is probably where they will remain. I can't see them selling for higher due to they aren't able to do anything special other than looking like a "black horse" lol.