An ermine is a small Earth mammal that can be described, much like a tribble, as being "nice, soft, and furry."
After Leonard McCoy made such an assessment with regards to the "practical use" of tribbles, including the fact that they "make a pleasant sound," Spock retorted that "So would an ermine violin, but I see no advantage in having one." (TOS: "The Trouble with Tribbles")