I think the encyclopedia was a grocerystore Funk & Wagnalls, which I realize is not one of our more respected reference works, but this one is just too preposterous to ignore. In distinguishing between apes and monkeys, it said that most monkeys have tails, while apes have either no tails or “very short ones”. The same article said that apes and monkeys formed the superfamily Cercopithecoidea, which I doubt that any zoologist would accept.
But where does this idea of tailed apes come from? Gibbons don’t usually figure prominently in discussions of apes that are aimed at laymen, so I’m thinking maybe there are species of gibbons that have caudal stumps. Are there? Or is this just another example of F&W sloppiness?