Here is a nightmare to inflict on anyone: Uncle Milton's Aquasaurs.  If you crossed pubic lice with the Highlander and Jeffrey Dahmer and made the resulting monsters live under water, they might be half as revolting as these things.

After a few weeks all you can see in the vat full of filthy water is two inches of sludge with little tracks moving through it where the triops are. When two meet there is a brief explosion of activity and only one track leaves. Sometimes bits of the loser drift out of the ooze.

An ex of mine bought these things for his daughter when she was just a little girl.  I was horrified and very very happy that, um, something went wrong and they never appeared to hatch.