Rookit

I did a little digging and apparently It happens after a player dies and respawns,

I'm playing on Andriod

And it seems to happen on multiple worlds

I'm Aware there's a delay from when the horse is mounted and eating  grass but the video is only 7 seconds becuase that all im allowed to upload but the original clip was about 30 seconds of the bugged horse with its head down and not appearing in the correct upward position even at pausing to let the frames catch up.

No, there are no mods or addons being used, a short video has been posted In attachments

I put a video in the attachment box