Skeleton Horses get injured

The version I played is minecraft for win10. I have cought four skeleton horses and keep them in my stable. And then they all disappeared. I load in the archice I have saved before and find that there health bar decreased for no reason.I am sure that they are not croweded because I had ride on one and took it to another place.But it was still dying.(p.s. I am a Chinese so my English is poor, sorry.)