A stallion is a male horse that has not been gelded (castrated). Stallions follow the . When erect, the penis doubles in length and thickness and the glans.
This chart is incorrect. It says a stallion can be up to 18 inches long. I have been on a lot of ranches and that would actually be at the smaller end of the sizes of.
Penis size (in all species) is primarily genetic. 20 year old retired International showjumper, who is a gelding that thinks he is a stallion.