The old saying about birds, pigs and rats keeping their own company, turned into a short rhyme with a wink at the end about the speaker having their own tastes too.
From Mother Goose / Nursery Rhymes (traditional). See the whole collection.
Birds of a feather flock together,
And so will pigs and swine;
Rats and mice will have their choice,
And so will I have mine.
About this rhyme
A rhyme built around an old proverb, birds of a feather flock together, extended out to pigs and swine and rats and mice for the sake of rhythm. It is really a rhyme about liking your own kind, dressed up as animal observation. Good for introducing the phrase itself, which still gets used well beyond the nursery.
Public domain. Text from The Real Mother Goose (Blanche Fisher Wright, 1916), via Project Gutenberg. View the source edition