Pretty simple, male or female?
Found this link.
Lucien Lachance specifically refers to Shadowmere being a she in his dialogue in Oblivion. The dialogue in Skyrim however says Shadowmere is male. So who knows? I was a huge Oblivion fan so I can't think of Shadowmere not being a mare, but it's up for debate. I'm with AASamurai on this one, linguistically why would you call a male horse "-mere" when it's clearly a cognate of mare?
Silly Bethesda, why don't you write this stuff down before making a new game? xD
maybe it is just a breed of horse. And this Shadowmere is the off spring of a long line of Shadowmeres that lead back to the very first Shadowmere. Think of it this way, the Shadowmere behave alot like the Phoenix who reincarnates itself in order to survive and give life to the next generation. Whether it be male or female. Maybe the Shadowmere behave in the same sense, except combustion into an ash ball, it reanimates itself with the black tar.