The Painter's Daughters with a Cat, Thomas Gainsborough, 1760-61, oil on canvas, 30" x 25", held by the National Gallery, London, England.
I always supply the link from which I obtained the image I reproduce, and that is the BBC. The cat in this painting is unfinished--look in the crook of the taller girl's left arm, behind the front girl's left shoulder. It is facing slightly to the right, as are the girls.
But I must implore you to also try this link to the owner of the piece, the National Gallery (UK), here. Their image is zoomable and moveable and really is the only way to properly scope out the unfinished phantom cat.
His two daughters are captured in a far from formal demeanor, in a natural pose that surely must have been familiar to him. The presence, barely hinted at, of the animal--which must have been reluctant to pose, judging from its angry expression--contributes to this recreation of domesticity seen through a father's eyes.
This is a fascinating painting. Much as I love cats, I don't see how including this cat in the image improves it. Perhaps that's why Gainsborough didn't finish it--he realized that the cat detracted, rather than added, to his daughters' portrait.