Whenever I saw Belfort, I remembered the Handsome (Aramis) with his "poor sheeps." They're both very sweet and would have matched characters.
The actions of both cartoons take place in France, at about the same time. The Musketeer dogs lived under Louis 13 in 1625, and Belfort after 1643 already under his son Louis 14, so let's imagine that at some point the characters crossed paths, being long-lived :D
By breed, Belfort is a French spaniel, and Handsome is presumably a greyhound. Maybe next time I'll draw Belfort as a puppy playing with an old Handsome.

I searched for all the information myself, so there may be inaccuracies :D

"Dog in Boots" and "Belfort & Lupin"