Variations
Frank Edwards, in his account of this story in Stranger Than Science, adds that some experts put forward the theory that the tracks were made by two or three creatures of an unknown type that just happened to travel in such a way as to make their individual trails look like one long one. It's a pretty nebulous theory, but one that helps support Mr. Edwards own theory of a connection between this event and the discovery of some unusual corpses on Canvey Island a hundred years later (See "The Canvey Island Monsters"). It's also interesting to note that, while stating the more spectacular features of this event, Mr. Edwards forgets to mention the year it happened.
In Charles Berlitz's World of the Incredible but True, Mr. Berlitz gives a very brief account of this event -- one paragraph -- yet he still manages to make a mistake by asserting that the prints went up walls, which they most definitely didn't.
The account of the event given in Strange Stories, Amazing Facts gives the date the tracks appeared as the night of February 9, 1855, rather than February 7 (thus, in this version the tracks are discovered on February 10). This version states that the winter was unusually cold, to the point the the Exe River froze over; if rivers were indeed freezing over, then whatever made the tracks may have had an easier time crossing the bay near Powdersham Castle than one would normally suppose. The starting point of the tracks is given as "a garden in the parish of Totnes," and the end point as a field at Littleham.

Theories
As mentioned, Sir Richard Owen put forth the theory that it was a badger (or pack of badgers) that created the trail. He pointed out that the badger places its hindfeet into the prints made by its forepaws when it travels; and, although they normally hibernate through winter, badgers do sometimes venture out for food in the middle of the season. It should be noted that Sir Owen never actually saw the Devonshire prints himself; he based his theory on illustrations of the melting prints by a friend. Obviously, to even consider his theory plausible, it must first be accepted that a single badger could make a one-hundred mile trail overnight, or that a number of badgers, operating independently, should just happen to align their searches as to create the illusion of a single trail.
According to Strange Stories, Amazing Facts, several other theories of a questionable worth were put forth. It's said that an amateur naturalist suggested that the trail had been caused by a kangaroo that might have escaped from a traveling menagerie and later returned with no one noticing. A vicar, the Rev. Henry Fudsen, is credited with a sermon that squarely placed the blame on the paw marks of cats.