So this is in Minecraft Windows 10 edition, 0.15.0. I don't know if the next update fixed this but I was too lazy to check.
I started up a new world on hard difficulty because I was feeling brave. Everything was alright. There were horses here and there and I was eager to get my hands on a saddle to tame them. On the first night, I found a dungeon. It was full of skeletons but I never destroyed the spawner in hopes i could make an arrow factory later. It was so close to the surface that when I barricaded and lit up the dungeon, skeletons started spawning on the ground above me. Long story short, it was a challenge to get out from underground when the sun came up.
I then went to build a house. I started but it would be a while before I finished. Rain started to fall and then quickly became a thunderstorm. While i was building, I saw lightning strike in the distance and then I saw a skeleton horse. I wasn't thinking of what it could have been so I went over and BOOM, Skeleton Trap. I died quickly. Journey after journey back to my house, i ran past a bunch of these things. Lightning corrupted almost every horse in the loaded world. The four horsemen of the apocalypse quickly became eight, twelve, sixteen, twenty. I was too scared to walk around anymore because I couldn't tell apart the horses the skeletons were on, and the horses that could trigger more of them. It was just rediculous. But now the storm cleared up and after several nights of frustration, I finished my house, bow shot all of them and now I have skeleton horses walking around EVERYWHERE.
(TL;DR) Skeletons, horse skeletons, lighting, disaster everywhere.