Japanese researchers just published a paper in the Royal Society journal Biology Letters titled "Disposable penis and its replenishment in a simultaneous hermaphrodite." Slugs typically have both male and female sex organs, and mate by coming together such that each male and female organ aligns with the other slug's opposites, donating sperm to one another.
Other species shed the male sex organ after copulation, however researchers believe that Chromodoris reticulata is the first creature known that can re-grow its male appendage.