Ah, I story I gladly share. Don't read if you're under 18 years of age.
Back when I was serving the nation as a civil worker in 02, my fello worker and I always used to read a q&a column of a sexual therapist in a newspaper. One day, a poor bloke asked what to do to make women stay with him. All his girlfriends said he wouldn't amke their hearts throb. Before reading the answer, I yelled out: "That's easy! Cunnilingus!".
Now my coworker wasn't that fit in latin, so he asked me what that meant.
Well, I did the best explanation I could: I spread my index and middle finger a little and stuck my tounge between them, which he commented with a "Bertl, du perverse Beutelratte!", which translates roughly to "Bertl, you perverted Opossum!".
Now, as "Beutel" means bag, he used it as a synonym of testicle and he himslef translated it into latin. Thus "Coleus Rattus" aka "testicle rat" was born.