When breeding horses together on my server, instead of receiving a colt, a full grown horse by appearance was made. The shadow of the horse was very small which lead me to suspect it was a visual bug of sorts. I went and got wheat in order to see if feeding it would cause it to mature normally and allow me to ride it. As soon as I clicked on it with a single wheat, the shadow grew to the size of a normal horse. I fed it 18 wheat and it wouldn't take anymore, so I attempted to ride it, only to be thrown instantly onto the ground without any interaction being made with the horse (ie. smoke puffs, neighing, bucking). I attempted to relog into the server, and when I came back, the horse's shadow was small again. I fed it an entire stack of sugar in order to confirm whether or not it was a colt simply appearing as a full grown horse. After feeding it the entire stack it allowed me to interact with the horse normally, confirming that it was indeed a visual bug causing colts to appear as horses. This has not only been true of bred horses, as there have been numerous wild horses that have been unable to be interacted with despite appearing as full grown horses.