Image credit here. The Supper at Emmaus, Jacopo Bassano, 1537, Oil on canvas, 92" x 98", held by Cittadella Cathedral, Padua, Italy.
Zuffi makes an interesting observation:
Cats are often present under the table in scenes from the Gospels painted during the Renaissance. It is not always necessary to seek symbolic meanings at all costs; sometimes they are simply realistic touches.
The cat in the lower left seems angry and confrontational, wondering what the resting dog may do. So sometimes a cat is just being a cat, even in priceless art of antiquity.