I accidentally rode off a mountain that was too steep with Shadowmere, i waited over 2 in game months now checking back and the horse is gone, any ideas? i'm not sure of the EXACT point of impact where he died but i've searched all over that mountain side with no signs, also not sure if it matters but after he died he fell into a stream and floated downstream...

where exactly did you look for him? He respawns where you first got him, suppose too anyways

If on PC type

player.placeatme 0009ccd7

into the console

He respawns where he died: http://www.uesp.net/wiki/Skyrim:Shadowmere

also, a detect life spell/shout would come in handy

I had heard that he respawns wherever you get him/dark brotherhood sanctuary or where he died... dunno if that's true though.

I hope not, he won't stop following me around. It seems like every time I come out of a building BAM! there he is. It's like take a hit guy you're creeping me out.