The weasel group in D&D includes ermines, stoats, polecats, minks, martens, otters, and similar small, thin, burrowing, occasionally semi-aquatic mammals. They are generally not a threat to adventurers (though their Dire versions may be), though their pelts may be quite valuable.
Familiars[]
Weasel familiars grant their speed and agility to their masters.
Otters[]
Otters are weasel-like animals that spend much of their time in the water.