"Mustelids" are a broad category for animals distantly related to weasels. This includes actual weasels, ermines, minks, ferrets, and similar creatures, but also otters, badgers, and wolverines. In addition, though it isn't sceintifically accurate, this category here includes skunks, tasmanian devils, and their kin.
Mustelids are far-ranging and diverse, often filling a niche as an aquatic predator of fish, but also eating nearly anything in the colder regions on land. They have strong jaws, and the land-dwelling species are usually burrowers, with powerful claws. Clever and tenacious, these creatures have a reputation for sly cunning, or for brute ferocious strength. Their fur is often valuable.
For D&D, mustelids share several traits in common:
Mustelids are tenacious combatants. Like most creatures, they would rather flee than fight, often darting into burrows or nearby bodies of water. However, if cornered, these creatures are relentless fighters, grabbing with tough jaws and holding on for dear life. The larger they get, the more dangerous they are to normal mortal life, with jaws that can crush skulls and break legs, and claws that can tear open a gut.
Mustelids have a reputation for cunning and agility by and large, and, though they are easily spooked, many weasels, ferrets, otters, and others certainly display it. Larger mustelids, such as badgers, skunks, and wolverines, are reportedly very ill-tempered and territorial, not afraid to take on much larger creatures in order to defend a den or protect a kill.
Mustelids are fairly independant, living on their own in large ranges, or in small clans and family groups where food in plentiful.
Many mustelids eat fish, and are adept at swimming to catch them. A few are more land-based, and these creatures are opportunistic feeders, eating anything and everything that comes along, and defending these kills fiercely. The fur of most mustelids is valuable, as it can be very insulating, and frequently repels water.
Dire wolverines are some of the strongest mustelids, being as they are the larger, more savage kin to the common wolverine, which is dangerous enough.