Hey, here's what I think is an even better idea. Have the horse 5' step backwards into the mist, turn invisible, and then
dismount IN FRONT OF IT. They'll sure as hell never expect that.
As long as you don't wind up right next to the hound, they should have a hell of a time finding you I think.
Square M or N 11 for that, I guess. That way, you won't be adjacent to either enemy, so even if the hobgoblin has Scent they still won't easily know where you went.