Animals despawning

I've had this world for almost a year now and I've tamed quite a few mobs and they've almost all despawned, which is really sad because I've spent a long time gathering them  and making memories with them, I would really like for this problem to be resolved and maybe have my mobs returned, and also I've had some troubles with horses despawning in the past but it was only that mob until now. Please help me Mojang :/