My minecraft pets keep disappearing, why?

I have lost 7 pets on one of my Mincraft (X-box edition) world. 6 dogs and 1 cat. They just disappear in the middle of my game. I feed them regularly so they don't starve and I always have them in some sort I shelter. It's on Normal settings and I often go out to kill mobs at night. Could that have anything to do with it?