Evolution has produced a leg-crossing array of different reproductive kit. Here are some of my favourites (see more here).
Most mammal penises come with the delightful sounding 'penis-barbs'.
These are spines around the shaft of the penis that either help the male
to, er, occupy the female, or to rake the female's *ahem* passage on exit in
order to stimulate ovulation. Either way, this explains why female cats
can sometimes be heard shrieking like banshees from hell.
Male snakes (and lizards) have the impressive accolade of possessing two penises each. Some think that symmetry is the best explanation here. That hypothesis works for marsupials like the opposum, where the symmetrically bifurcated penis matches the symmetrical anatomy of the female (two vaginas!!! How come this meme never features in porn? Wait, maybe it does.) Mummy snake, however, just has the regular one front-bottom. So how come the males are packing spares?
The likely explanation lies in the fact that snakes form mating balls. As a snake-phobic I can only indicate how monstrous this phenomena sounds. A single female gets accosted by a huge number of males at once, and they writhe over one another attempting to get their end away. Such stiff (hehe) competition calls for epic sperm production. By using just one of his two peni (i made that up) at a time daddy snake can put the other one on recharge, and so hope to flood out his competitors semen.
Echidnas take the prize for the most alarming tackle. They have four-headed penises. Females have two vaginas, and the males ejaculate from only two of their four penises at once.
Male mallards have twisting, corkscrew shaped penises to match the corkscrews of the females reproductive canal. It is thought that both are the outcome of an evolutionary arms race, in which females have undergone selection for increasingly erratically shaped reproductive canals because they make it harder for the female to be raped, thus maximising her control over the quality of her mates. Male mallards are notoriously violent, with females frequently being subjected to gang rape, and sometimes drowning to death in the process. It seems fitting then, that the males should have to suffer what I'm assuming is close to every man's worst nightmare: at the end of each mating season their penises shrivel up and drop off.