My favourite is Baboon Night, which takes place at the Houblon Arms in Oasby, Lincs. It commemorates an 18th century event in which the heir to the local manor, a small baby, was kidnapped by a pet monkey and dropped off the roof of the house. Every year, the monkey is ritually punished. There's a twilight procession (involving a man dressed as the monkey), and the effigy of a monkey is thrown over the roof of the pub, at which point the crowd is allowed inside.

Apparently this tradition is about 15 years old.