Why would Bethesda name their dark horse after a pony from a show nobody really cares about? (IIRC, Shadowmere was created in Oblivion, which was made in 2006, way before MLP even got popular)
Besides, Dahs does not sound like Dash in any way. I think this is a case of straw grasping, but that is my opinion.
I might be just hearing things, but I swear every time I say Shadowmere it sounds like Rainbow Dash when played back at me, and yea and I questioned why Bethesda would do something like that too, it does seem far fetched. Then again Rainbow Dash was around before G4 so....i'dunno, im not sure if she was around before Oblivion though.
Edit: P.S I also used the wrong choice of words saying it was a hidden message, should have said odd little possible Easter egg? I dunno, just thought it was really intresting.