After upgrading from Minecraft 1.10.2 to 1.11 and all upgrading Java from the previous (latest) version to 1.8.0_111, and logging back on to the game, the horse that I had been mounted on when closing the game (under version 1.10.2) disappeared. The log contains the following lines:
[07:51:02] [Server thread/WARN]: Invalid statistic in .\world\stats\8365f473-a478-4cf1-aaab-6e15862dd350.json: Don't know what stat.killEntity.EntityHorse is
[07:51:02] [Server thread/INFO]: sparetimer[/192.168.100.7:55147] logged in with entity id 182 at (-1854.6217083986073, 71.8500000178814, 285.74785887473524)
[07:51:02] [Server thread/INFO]: sparetimer joined the game
[07:51:05] [Server thread/WARN]: Skipping Entity with id minecraft:entityhorseI backed up the 1.10.2 version copy of the game's \world folder before running 1.11, let me know if you need a copy to repro.
Cheers
Wes, you can rescue your horse from the playerdata backup with NBTExplorer (or similar) by changing key 'RootVehicle/Entity/id' from "EntityHorse" to "horse" or "donkey", depending on if there's an 'Items' key in the same level.