Hrair is a word found in the rabbit vocabulary of the book Watership Down. Since rabbits have an awful time with numbers, they only have actual words for numbers one, two, three, and four. Any number after that is hrair, which literally means "a thousand", but often means "a lot".
U hrair, or "The Thousand" refers to all the enemies, or elil of rabbits, which include the fox, stoat, weasel, cat, owl, etc.*
Fiver's real name in the book is Hrairoo, which means "little thousand". Being one of the last in his litter, although obviously not the thousandth, his name refers to the fact that he was "the runt" of his litter.
* although the book includes men in its definition of elil, later, Bigwig and Cowslip mention men being a seperate group from elil.