I can't remember the name of the fanfic but in a strange turn of events (which i can't remember how it happened but probably had to do with Loki's magic or Heimdall's messed up gate) Thor and Loki end up on a strange planet (I don't think it was earth but it might've been). They only had a horse (which i think they found) and a couple items. At one point of time Loki is forced to follow Thor (who was riding the horse) because his hands were chained. Another time Loki was the one riding the horse because he got hurt. (wether it be from 'rated m for mature' reasons or because of a fight.