The Invisible Pink Unicorn is the goddess of a parody religion – a unicorn which combines the mutually exclusive properties of being pink and invisible at the same time.
It was first mentioned in the alt.atheism
newsgroup in 1990, to satirize the fact that it's impossible to disprove the existence of deities outside of common human perception.
Followers of the IPU believe that it "raptures" socks, which is the reason why they go missing. The IPU punishes disbelievers by poking them with her horn – a phenomenon usually, but incorrectly, attributed to mosquitos. There's also a huge controversy on which kind of pizza the IPU likes. Everyone seems to agree on pineapple, though.
A commonly used ritual phrase is "Blessed Be Her Holy Hooves" (bbhhh).