According to the OED’s etymological discussion, the form pentthous is a spelling that reflects a folk-etymological re-interpretation of the word as pent (‘slope’, possibly referring to a sloping roof, cf. the much later word pent n.2) and house. This suggests that Anglo-Norman pentthous is presumably a borrowing from English (where this type of spelling would become prevalent), even though its use in gaunt¹ antedates the first English attestation by well over a century (the OED’s earliest attestation is from 1530).

Continental French only has the non-aphetic forms appentis (DMF) and appendis (DMF) with the same sense; cf. Anglo-Norman apentiz.