I think the guy looking for his horse was Negan. And of course Morgan stopped Rick from shooting him. Another reason I think this is because of the big speech (good one actually) where Morgan tied his mercy on the Wolf boy into it leading to the saving of Carl's life. They're setting that up to be ironic when not killing Negan leads to the death next episode of Michone and/or Glen and/or Darryl. I don't think it'll be Rosita.

They also showed a guy riding bold as brass down the middle of a street on a horse in the previews for next week. I don't think that means the guy is just some schmo on a horse. He's someone important. At this point who else would it be?