I had a group of four skeleton horses in a pasture, a fenced in, 8x8 area. Each of them had different name tag on them. I left the area to go to a different section of my world only to see that there were now only three horses left. The fence was complete around them, no blocks for them to jump on, and I had no one else on the session.

I was playing on a pure survival world, on Peaceful at the time, and on the most recent release of Minecraft.