wild on a common
The modern locution might be "like a horse in a pasture." A "common" was the common ground set aside or simply taken over by a community for pasturage of the small flocks and other animals belonging to its individual members. In the two generations previous to the writing of Frankenstein commons had all but disappeared in the British Isles as a result of their "enclosure" by landlords. The effect was enormously disruptive of rural stability, as Oliver Goldsmith's well-known poem, The Deserted Village (1770), recounts.