Why do some horses chew their buddy's tail?

It's a big problem - a horse comes in with its tail chewed up, and a pasturemate is responsible. Why do they do this?

It's a bad habit that's most commonly caused by boredom. It's most common in foals (who get bored easily), but definitely happens in adults. The culprit I know is old and retired. It can also mean they need a mineral supplement. It can be cured by separation (except that can cause other bad habits) or putting something foul tasting on the oh-so-tasty tail. You can identify the culprit usually - the only one without a chewed tail (although the one I know only chews the tail of one of his pasturemates...for some reason. That one *points down*. Poor CeCe...