Animals such as axolotl and fox keep disappearing.

Animals such as axolotl and foxes keep disapearing I have had to replace my axolotl 3 times and they keep disapearing. I have them in water at my house in a 4 by 3 area I had them named as well so thought they wouldn't despawn. I had breeder foxes and brought the baby back to my house on a lead. I put the lead on a fence in my house but when I logged on today the fox was gone but the lead was still on the fence. No one had been on the server and their axolotls keep disapearing as well.