The 1790 and 1800 Censuses for Wells provided a count of 1 under "All other persons except Indians not taxed" in the household of John Wheelwright. This may, of course, have been a different person in 1800 than in 1790, but it's assumed here that it was the same person. This may have been a person who was formerly enslaved.

Might this "other" person be Zilph, who was brought to Wells in 1764 by Joseph and Mary Wheelwright, or Charles, who was brought to Wells by Nathaniel and Abigail Wheelwright as an infant in 1744?

Bibliography:

1790 and 1800 US Census for Wells