Ok, so I was breeding my horses. I gave them both a golden carrot, after that it lagged a bit and then an adult horse appeared. I tried to ride it but it wouldn't let me and it had the same sounds as a foal. I didn't think to screenshot it (sorry)
Update: I right click on the horse/foal and it redirects my curser to a random spot on the screen. Plus my whole game needs a second to load every 30seconds or so.
Update: Happened again. Same colour as the previous glitched horse (grey dapple)