Peacocks have one of the most famously oversized and colorful tails of all birds. They are relatives of the pheasants.
Peafowl (Pavo cristatus) in the Toronto Zoo.
The Indian Peafowl is the national bird of India.
Peafowl do not have a pleasant voice and and these days people don’t eat them, even though they are edible, and have been consumed in the middle ages by people who just had to show off that they can afford it.
The peacock’s main claim to fame is his extravagant tail (actually a train of cover feathers, not really the actual tail feathers) which made evolutionary biologists wonder how could such a thing have evolved by natural selection when it is an advertisement for predators where to find its owner if they want a snack.
Thus the word a peacock’s tail has become synonymous with some useless but costly appendage whose only purpose is to show off in front of the opposite sex — and a shorthand for saying sexual selection (follow this link to see a more detailed description of what that means).
Further Readings:
Indian peafowl on the Toronto Zoo website.
Indian Peafowl on Wikipedia.