Elephant would be interesting. I still remember watching my uncle's racehorse breed a mare when I was about fourteen. Call me a pervert, but I don't think my dick has ever been harder. His tool was about two feet long, veins about to burst and the veins on his head [his temples] about to burst too. He shot so much cum that some of it gushed out of the mare's vagina.
If I was a horse breeder I probably wouldn't remember it so vividly. That stallion was a stud. That stud was a stallion. Whichever!