Actually a combination of two different breeds, the Ynysfor and the Old English Broken Haired Terrier, the two breeds were brought together and became the Welsh Terrier. Today, they look like a miniature Airdale and were introduced to movie audiences first in the 30s with Asta, in the Thin Man movies.
Mischievous is a good adjective for these guys. They are clowns who like attention. They absolutely require good mental and physical workouts. If they don’t get enough stimulation, then they begin to problem solve in sometimes undesirable ways, like how to open the fridge door, for instance. They may also bark and dig, too, if not stimulated enough.