I had 3 in a 29 gallon, 2 were unhealthy and sadly died after 3 motnhs, my current one is doing great, but I'm not going to be able to get horses for about 4 months and would prefer to downsize. I finally convinced my parents to let me get a 15 high for it and some dwarves as I figured that would help with the socializing. It's a northern erectus btw so it's quite a bit smaller than the southern ones.