John "I am not from Denmark" Foss and his only wife Mary Berry
In his books on the Foss Family, Guy Rix, a blacksmith in Concord, New Hampshire, pieced together the genealogy through correspondence. This was done in the early 1900's. Apparently a Foss family in then Danish Virgin Islands wrote to him and told him of their Danish ancestry. For whatever reason Guy assigned their ancestors to all Foss's in America, which is simply not the case.
Guy Rix then combined the two men, added the fake Danish ancestors gave it all a good stir and left us with a Foss fiasco. Now some people really really want the Danish Ancestry to be true, but from what I have read David L. Foss had twins born in 1638, both girls. Rix substituted "Johan" for one of the girls names and Voila Danish Royalty added to the family. As always, if you think that I have any of the following information wrong, please point it out, and please include your documented sources. So here is what I can piece together about John Foss of Rye and John Fost of Dover.
In his will proved 13 September 1667 Humphrey Chadborne of Kittery, Maine wrote a very lengthy will in which he left 5 pounds to his "cosson Mary Fosse". This confirms the link between Mary Fost and the Chadbourne family. Humphrey was likely her uncle, despite being referred to by his as his cousin. 
death of mary
Mary Chadbourne was alive on 11 March 1673 when she gave birth to her son William. He had his birth recorded in the Dover records in the 1700's after the birth of two of his own children. John, her husband was know to have been remarried to Elizabeth Goss by 1688 when her husbands estate was probated.  Most researchers place Mary's death closer to that date. But here's an interesting item in the court records. In June 1674 John Fost and his wife were presented in court in Dover for have a child only 12 weeks after marriage.  As there are no other John Fosts in Dover and John Foss of Rye was still married to his wife Mary Berry, who else can this be?
So, our first John Foss/Fost should look like this:
John Fost of Dover, b. Unknown, d. 1699
Married (1) Mary Chadborne, perhaps by June of 1674 (2) Sarah (Crawford) Goss
John Foss of Rye should look like this:
John Foss, b. 1633 Paignton, Devon, England, d. 1710-1716
Married: Mary Berry, d. between 1707 and 1710
John b. abt. 1660
Elizabeth b. abt. 1666
Samuel b. abt. 1668
Martha b. abt. 1672
William b. 11 March 1672/72
Zachariah b. abt. 1678
Hannah b. abt. 1682
 Albert Stillman Batchellor, Otis Grant Hammond, Ezra Scollay Stearns, Probate Records of the Province of New Hampshire, vol. 1 1635-1717, State Papers Series Vol. 31, (Concord, NH: Rumford Printing Co., 1907) 450, digital images, Archive (https://archive.org : accessed 26 December 2015).
 Batchellor, Hammond, Sterns, Probate Vol 1, 320.
 Dover Historical Society, The Collection of the Dover Historical Society, Vol 1, Births and Marriages,(Dover, New Hampshire: Scales and Quimby, 1894), 40, digital images, Archive (https://archive.org : accessed 26 December 2015).
 William Richard Cutter and William Fredrick Adams, Genealogical and Personal Memoirs Relating to the Families of the State of Massachusetts,Vol 4, (New York: Lewis Historical Publishing Co., 1910) 2461, Archive (https://archive.org : accessed 26 December 2015).
 "Maine Early Wills and Deeds," database with images, American Ancestors (www.americanancestors.org : accessed 27 December 2015), citing York County, Maine Wills, Registry of Deeds 2, 27, entry for Humphrey Chadborne, 1667.
 John Scales, Historical Memoranda Concerning Persons and Places in Old Dover New Hampshire, (Westminster, Maryland : Heritage Books, 2007), 381-2.
 Collections of the Dover New Hampshire Historical Society, Vol 1, (Dover, Scales & Quimby, 1894), 40.
 Albert Stillman Batchellor, Otis Grant Hammond and Ezra Scollay Sterns, Probate Records of the Province of New Hampshire, Vol. 1, 1635-1717, State Papers Series, Vol. 31, (Concord, New Hampshire : The Rumford Press, 1907), 320, digital image, Archive (https://archive.org).
 Otis G. Hammond, New Hampshire Court Records, 1640-1692, Court Papers, 1652-1668, State Papers Series, Vol. 40), 304, digital images, Archive (https://archive.org).
 John Plumer, "Two John Jacksons from Dartmouth", The New England Historical and Genealogical Register, The New England Historic and Genealogical Society, 144 (January 1990); digital images, American Ancestors (www.americanancestors.org : accessed 26 December 2015), 33.
 Batchellor, Hammond, Sterns, Probate Records, Vol 1, 220
 Henry Cole Quinby, New England Family History, A Magazine Devoted to the Families of Maine and Massachusetts, Vol 3, New York City : H. C. Quinby, 1907), digital images, Archive (https://archive.org : accessed 26 December 2015), 432.
 Rockingham County, New Hampshire, Land Records, Vol. 13, p. 94; digital image, Rockingham Country Register of Deeds, (www.nhdeeds.com/rockingham/roindex.html), search > Foss, Jno.
 Rockingham County, New Hampshire, Land Records, Vol. 9, p. 419; digital image, Rockingham Country Register of Deeds, (www.nhdeeds.com/rockingham/roindex.html), search > Foss, Jno. > 419> John to Joshua. This deed was written 13 September 1710 and recorded on 28 May 1716.
This blog post updated: 27 December 2015