An ermine pictured in Lady with an Ermine
An ermine was a small Earth mammal that could 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 in 2268, 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")
The painting by Leonardo da Vinci, "Lady with an Ermine", featured a woman holding this creature. (VOY: "Scorpion", "Concerning Flight")