If anybody has lost their shadowmere make sure that the horse you last mounted isnt alive.. and Shadowmere will appear on the road above the brotherhood sanctuary.. but only after you killed the horse i had an owned horse near whiterun and never realized it.. i spend hours looking for shadowmere thinkin arvak was the problem.. all i did was check every stable in every hold.. if you lost shadowmere after summoning arvak outside of soulcairn then shadowmere will most likely be wherever you last fast traveled too before mounting arvak.. if you mounted arvak inside soulcairn he should be either at castle volkihar or by the boat before crossing the river to the castle.. if he dies.. he should respawn at the sanctuary after a very long waiting time about 30 days or so but becarful where he's killed as some areas are glitched and have no spawnflags.. just thought maybe this would help if he's ever lost on the 360 version