Disappearing Blocks and Mobs!

I was excited about the horse update when it came out, and I set out to find two horses in my survival world. It took hours of searching because I had been already exploring the world so all the chunks were too late to update. After wandering far north-west of my home I found a herd and leaded two of its members to bring back(The leashes kept breaking between lag spikes if I didn't stop for them). After following the map I was using to bring them home I quickly made a makeshift fence out of dirt and put them in there. I left the game to go to bed before taming them, so I expected I could do so tomorrow.

I woke up today to play Minecraft and start to tame, and then breed, horses, but one of the horses was missing! I restarted the game again and again, then I dug underground to see if it could of glitched into a cave under it. I even retraced half of my steps to see if the horse was somewhere the lead had broke on my trip back home. Despite my efforts to "un-glitch" the mob back into the game, its still missing.

Steps to Reproduce:
1. Spawn, move, or place mobs/blocks in a certain area.
2. Quickly leave the game.
3. If you're unlucky enough, a valuable mob/block placement will be gone forever.

Please help, this glitch is common for my Minecraft. It sets my progress back a ton if i'm not careful!