I cant find him/her/it. I dont even remember where I last was with them is there a way for me to summon him/her/it?

Bump as this is of great importance.

http://elderscrolls.wikia.com/wiki/Shadowmere_%28Skyrim%29

EDIT: I think I misunderstood, you already have Shadowmere, but you lost it?

If you fast travel he'll appear with you (or go to the stables if in a city). If he's been killed he'll respawn (10 or 15-20 (Wikia and UESP) game days later) at the location he died. Of course, I guess it's possible for him to run off to chase something if you enter the cell even if you're not close enough to see him.

Edited to correct respawn time.

FreeWilly, I have the same problem as you do on my first character (who's almost unplayable due to lag); Shadowmere disappeared at around 210hrs playtime and at 270hrs she still isn't back. I'm guessing that for you, she's probably stuck, dead and in the process of respawning (check back at the Sanctuary) or missing permanently.
No way but to wait for her, I guess.

Thanks for all the suggestions guys i'll be sure to try them all out and update you if a solution is found.

If shadowmere died then he is gone forever.

00032BF5...This is her console ID (unless I was lied to).

Being a follower of sorts, perhaps a "player.moveto 00032BF5" or "00032BF5 moveto.player" command may work?--or not. I have not tried it on her so I don't know. Can't hurt to toy around with it.

She always shows outside of a cave for me whenever I fasttravel, or else at the local stable. Fast travel around to a few caves and see what comes up. And, yes, she's a kickbutt fighter that can climb stairs and fears nothing--not even dragons. Jordis left the game, so I tried her console number coming and going. It'd take me to where she "should" be at the house in Solitude. Do it the other way, and while the console command was accepted, she was not standing next to me as she should have been...gone forever. You may find the same thing...this game does have some "quirks". (530 hrs so far).

I think I read somewhere that he will spawn at the lake he was found at after 10 game days.

Sorry--gave you some bunk numbers. Use "player.moveto 0009ccd8", without the ", then hit enter...if she still exists, this will take you to her side. I just tested it (which I should've done before, but got tied up taking pics of the Transit of Venus).

I have heard the same thing as DarkObsidian, but, I still have a few in game days to go before I can confirm.

I killed him. After all, what's an assassin supposed to do. Besides, I didn't want to be associated with anything that came from that traitorous [censored] Astrid. Betrayed me (although I saw it coming a mile away), betrayed her family, betrayed the brotherhood, and trampled over everything they've ever believed in. Good riddance to her and her filthy horse.

BTW, @ OP: I don't understand how you could lose Shadowmere, the short time I had him, he followed me everywhere...especialy when I fast travel. I you try fast tarvelling and he stil doesn't show up, you can try waiting a few day (long enough for cells to re spawn, or chalk it up to him being dead.

There seems to be a bug where she literally disappears from the game. Bethesda, if you are reading this PLEASE patch! I miss Shadowmere

My Shadowmere disappeared for awhile and was replaced with a horse I'd bought some time back. I just kept playing and after awhile, I fast travelled to Windhelm Stables and both the purchased horse and Shadowmere were standing there. I got on Shadowmere, and s/he hasn't disappeared again since.

I haven't seen shadowmere for twenty plus in game days...

I've tested his respawn time after he dies, and I got 30 days. And he is a he. Lucien was a clueless noob when it came to horses.

Say what?
Assassins do not kill without reason, nor was Shadowmere Astrid's "filthy" horse, she just happened to be the current owner at that time. Shadowmere first belonged to Lucien Lachance, 200 years earlier. Shadowmere belongs to the dark brotherhood as a whole, he/she is a servant to sithis.

i never rode shadowmere after he apeared. i went back over 100 gameplay hours later and it was still fighting a penitus occulatus agent

Dammit i have the same problem. Shadowmere is missing and it's not spawning, appearing at the stalls i fast travel to, i do not remember when or where he dissappeared...

I've already stated my reason, besides, he's not just an assassin, he's already guity of 31 murders (not including db contracts).

Astrid being the current owner of that filthy horse makes it her filthy horse, and my character would have no part of of that traitor...including her filthy horse. It is a ROLEPLAY after all...my ropleplay, and I will play it as I see fit, and if I think she's a traitor, and think her filthy horse needs to be offed, then that's my busniness, not somebody else's. Understand now?

I'm not sure I understand your point. You should try to underline more next time to make your point more obvious. (;

But seriously, I just quoted the lore, no offence meant.
With that said however, it is hard to kill something that is immortal. :3

Really? You mean it was an only illusion when he fell over dead after I planted my axe in his skull...repeatedly?

Oh, he'll be back from the void in no time. All alone and sad in whatever place you decided to cut him down, wondering why his new master would do such a thing. Poor Shadowmere. ):

Shadowmere died fighting at an outlaw camp. I'm really not sure how he died and thought I lost him. I then went back by that same bandit camp and found the body. Making a note of where the body was, I kept on about my business. I visited the DB and he wasn't at the pond. Finally, I went back by the outlaw camp again after many game days of play and there he was......right about where he was killed.

Glad to have him back! He's a good friend to have when shtuff hits the fan.