Cats have what are called <a href="https://en.wikipedia.org/wiki/Penile_spines">penile spines</a> and when you look at it its a bunch of little spikes on it.

Duck penises do appear to have an arms race with their vaginal counterparts. The ballistic penises get better at penetrating while the vagina gets (seemingly counterintuitive) more difficult to penetrate. At least according to what I read, truly interesting!

Basically the duck vagina has a bunch of twists and dead ends and the female is able to open up and close certain paths to prevent the penis from getting far enough in, the male penis fights this by getting faster in order to by pass the "vaginal gates" before they close.

Now, lets just hope I don't put on some bestiality watch list for searching this stuff up.