Some animals and insects, like squirrels and bumblebees, use a “mating plug” to prevent other males from inseminating females they have already mated with. – WTF Fun Facts
Source: http://www.bbc.com/earth/story/20170419-the-largest-and-smallest-sex-cells-on-the-planet
inseminate
WTF Fun Fact – Flatworm Selfing
Flatworms have male and female sets of genitalia, and if they can’t find a flatworm to mate with, they will inseminate themselves by stabbing their head with their needle-like copulatory organ, a practice known as selfing. – WTF Fun Facts
Source: https://www.cnet.com/news/this-flatworms-freaky-sex-life-will-freak-you-out/