Mobs randomly disappearing

Me and my friends are playing on realm plus. I've been noticing many mobs randomly disappearing. I first noticed one of my horses with a diamond horse armor disappeared. Then once in a while when we log on we notice some of our animals disappeared. These animals includes cows, sheeps, and also my horses. Today another one of my horses who has been NAMED is gone. So I went to check and found our Dinnerbone polar bear is also gone. I also noticed all creepers who leaves my sight will immediately despawn. Please help me I love my first horse I can't lose him.