In Meung, a man in the tavern makes a comment about D'Artagnan's horse. What did he call the horse?

He calls the horse "a buttercup"

The stranger is clearly making fun of D'Artagnan's horse, because of the way his friends are laughing in response. When D'Artagnan hears the stranger call his horse a buttercup, referring to the odd coloring of the horse, he lashes out and starts a fight with the stranger.