Yes, I see now the 1891 Canada Census which matches what you said, that my great-grandfather Egerton R. Nix ("Nicks" in that document) was the son of Jesse (not Isaac) and Clarinda, with brothers John, Charles and what looks like "Ugene"... This certainly fits with the other records better.
This is the snippet that has thrown me off for years (and I can't find it anymore online, but you used to be able to find it by searching Yahoo for "egerton ryerson nix" or something similar)... Anyway, there were a bunch of records of Canadian marriages in the late 19th century on this website, and I saved them all to my hard drive.
#011023-1898 (Ontario Co): Egerton Ryerson NIX, 22, farmer, Prince Ed Co, Tp Reach, s/o Isaac & Clarinda NIX, married Annie Aleda HOUCH, 25, Tp Reach, same, d/o Nicolas & Mary Ann HOUCH; wit: Eugene NIX, Seagrave, Mary BUNKER, Barrie. 29 Dec 1897 at residence of Mrs HOUCH, Reach
All of the other mentions of Isaac Nix have his wife named Mary, not Clarinda (for children born both before and after the date of Egerton - either 1874 or 1876 depending on source). Also, the Eugene Nix mentioned is apparently his brother.
Family lore is that Egerton left Canada under shady circumstances, although my guess is that he changed the spelling of his first name so that Americans would pronounce it correctly, and not like "egg". (His son, my grandfather Edgerton D. Nix, was known as Ed or Eddy.)
So now, of course, my tracing of my ancestry beyond my great-grandfather is incorrect! It was Egerton, Isaac, Jacob, Hermanus, Hermanus, Peter, with notable variation in spelling of the last three names. But where did Jesse (or Jessa?) come from? I'm guessing that he was related to Isaac somehow, as part of the Ontario Nix clan, but I haven't found definite records for him...
In the 1891 census, he is shown as "Jessie Nicks, 46" (b. about 1845), listed as born in Ontario.
In the 1871 census, there is a Jessie Nix, but he's 32 (b. about 1839).
In the 1851 census, there is a male child which ancestry.com interprets as "Jessa Nix", although I have trouble making that first name out, who is 3 (4 next birthday), so born about 1847 or 1848.
So, it seems unlikely that these three are all the same person, but I don't see another similar person, and the Jessie I'm looking for was born in Ontario, so was presumably there for the 1851 and 1871 censuses?????