When you summon horses, if the baby ones have enough momentum, they can get stuck against walls, particularly in corners.

How to reproduce

1. Stand close to (but not completely against) the corner of a tall wall of solid blocks. A tall tube with a 3x3 interior seems to work for easiest results.
2. Using a spawn egg, summon horses against the corner.
3. Occasionally look down to see if any horses got stuck, the babies and teenagers will only be the ones stuck.

Additional notes

Adults are unable to get stuck. If you feed a baby until it grows up it will get unstuck and fall.

They can get stuck against walls, its just easier in corners.

If you push them they will unstick, but if they have enough momentum they will just stick to the next wall they hit.

Attached image shows some horses stuck in the tube described in step one. 

Attached video shows me pushing them