Am not researching either Forbes or Forbis exactly, but as neighbors of my husband's ancestors in Richland and Ashland Counties, Ohio, they keep showing up and am corresponding with a descendant. So, I'm noticing that in some records, the surname is spelled Forbis, but later is Forbes. These are Scottish immigrants, and the surname there seems to be Forbes. So, folks might want to be alert to the possibility that immigrants to the US might have been heard, by someone else creating a record, as saying Forbis, when they themselves spelled their name as Forbes (assuming they were literate and the spelling was well established, neither of which I can say at this point.
The records in question, showing this family as Forbis, are an 1853 tax record for Butler Twp, Richland Co, Ohio. Section 7 http://www.rootsweb.ancestry.com/~ohrichla/TaxMapCD/Butler18...
and the 1850 census for Clear Creek, Ashland Co, Ohio, both showing the name as Forbis. Later this will be standardized as Forbes, including on gravestones and later census, platt maps and tax records.