I signed up for the companions quest. I have to slay the stables guy, but he been dead and ashes from long ago. hen I stole his horse.

Can't I cancel the quest somehow and then sign up for Vikes quest?

Lol questing bug. I suppose you'll have to reload? Or you can search on the wiki for console commands if you're on PC, there's some way to cancel quests i believe. But the best way would be to reload probably.