The Argonaut is a type of octopus that has a detachable penis!
This type of octopus has very pronounced sexual dimorphism. This means that the male and the female of the species look very different. Think, for example, how male and female peacocks look very different. In the Argonaut's case, the male rarely grows to be larger than 2cm, while the female can be 5 times larger at 10cm. In fact, the dimorphism is so pronounced, that the female Argonauts were known since ancient times, while males were only described in the late 19th century.
Perhaps a more curious fact about this animal is that the males have a special arm called the hectocotylus, which they use to transfer sperm to the female. What they do is that the male's arm is inserted into the female, and then it's detached from the male!
The female literally cuts off the penis of the male when they mate. Until the males were discovered in the species, people thought that the hectocotylus was a parasitic worm!