So many people love their fantasy land. This isn't a huge shock. Myths about the basics of human anatomy have been around for millennia. Often, what contributes to them is basic carnyism. Little tricks guys will employ to make everyone think they have a giant dick. The most common one I've seen is the fluff-chub or semi-showoff. The guy gets himself semi-erect, to the point where it's mostly inflated but still hanging. After that, he will ensure some friends "accidentally" see it or, in a social environment deemed safe, whips it out. Everyone assumes it's fully flaccid and that it must surely double in size when erect. When, in fact, the guy would not get appreciably bigger.
Every guy who has worked all day out in cold weather knows their dick will practically hide up inside of them, and we all know that if you take a warm shower and play with it, even a bit, the thing can inflate to almost full size yet still hang. Instead of realizing their friend's BS, they try to convince everybody that their buddy is a horse.