Mobs like chickens cows sheeps etc despawn at a small distance

I found a wolf that is chasing a fox and I want to get the bone from my house and then when I go back they are gone and I went around that area and they are not there. Also when I found lots of while trying to find the wolf, I keep finding the wolf and the I give up and then when I go back to the place where the chickens are they are gone just like that. Why did this happen dude!