My tamed donkey disappeared and don't come back anymore

I just enter in my game in that moment I was mounting my donkey, then I just leave the game opened a moment and when I enter again in the game, I have to enter again in my world, but I wasn't in my donkey, just desappeared with all my stuff in it, then I close the game and enter again, I browse in a copy of the world in mode creative and just dissapeared my animal. I hope please can fix that cuz ruin my exploring in that moment and I have to do all again for that error, thank you.