The papillon ("butterfly" in French) is a delicate toy spaniel that became popular in European courts around the 1500s. Often called "comforters," a term common to any small pet lapdog, one of their acknowledged uses was to provide something nice and warm to hold or to sit on your feet in a drafty palace.
They also provided an uncomplicated source of affection in a world where almost every human being around had an ulterior motive. (When you think about it, being a princess must have been awful.) Dogs, an easily recognizable symbol of fidelity -- hence the name "Fido" we all recognize --began popping up in royal portraiture to show a woman's loyalty to her husband, or larger dog breeds served as foils to a ruler's military prowess. The papillion, being so elegant and tiny, and easily acquired only by certain high circles, became one of THE dogs for gentlewomen to have.
The Papillon Club of America has done excellent art-history homework to prove it. They have created a gallery of Old Master paintings which include Papillons, and most enjoyable it is too.