Flare

Flare

A stallion’s flare is the head of his penis, which enlarges 3-4x its original size when he has an orgasm. The shape is almost akin to a trapezoid at the tip of his penis, with the widest part furthest away from the horse’s body. Unlike the male dog’s knot, a flare will not stay engorged for long periods of time.

The reason that this trait developed is believed to be so that the stallion’s enlarged glans will trap semen inside of the mare’s vagina while he breeds her and the two remain ‘locked’ together for a few moments (although nowhere near the length of time that dogs are able to tie). If the penis is pulled out before the flare subsides, it may injure both the male and female.

References

http://beastiality.club/beastiality-club-extreme-animal-sex-content/the-encyclopedia-of-zoophilia/