Right, this should be a fascinating matchup. The AP of the two characters are a lot close range than I figured they were, with Otus having a slight lead in power. The biggest hurdle for Otus to face is Horus, due to the invisible nature of Stands, however since Pet Shop's ability is one of the few that can be seen clearly by non-Stand users, this should give Otus a much better shot here.
As for how he'll be able to approach the fight, he'll probably be able to dodge or deflect the initial icicle barrage from Pet Shop, and upon seeing that he's dealing with ice, he'll summon the only friend that would be natural in a battle such as this, Alphonse and his fire-laden shotgun. And honestly, with Otus' ease of flight, there's almost no chance of him getting frozen to the ground by Horus unless he's knocked down somehow. Unfortunately, the shotgun does take a fair bit of time to charge after firing, so he could summon Geddy to provide cover fire. He could also try Twig, but Pet Shop is easily way too nimble to be caught by Twig without some difficulty.
This is honestly tough. Real tough. I can honestly see this going either way, because while Otus has a varietal advantage over Pet Shop, especially with Alphonse on his side, Pet Shop is a gifted strategist and could easily think of multiple ways to outmaneuver Otus. And Otus can only deflect projectiles he actively swipes away, which means that he can take a serious hit if he's not fast on his feet.
However, if I think of who could win more times than not, I can see Otus pulling out ahead, if just barely. Alphonse can take a couple of hits if he has to, since he's a robot, and his shotgun could even melt Horus' ice in midair. Twig is excellent for mobility, allowing him and Otus to zip all over the place, and if he does somehow manage to get Pet Shop tied up, Otus can take full advantage. This is one of the closest matches I've come across yet, but I'm giving this to Otus.
Winner: Otus