In modern Greek, singular: χταπόδι, plural: χταπόδια.

Transliterated using standard ELOT (that maps χ to ch) singular: chtapodi, plural: chtapodia.

The word is composite and contracted. First part originally is οχτώ (8) (transliteration: ochto) but has been uncommonly shortened to χτα (chta). Second part is the word for foot (singular: πόδι/podi, plural: πόδια/podia).

So without the uncommon shortening in more archaic Greek it would be: οχταπόδι (ochtapodi) and οχταπόδια (ochtapodia).

If ELOT is ignored and οχτώ is transliterated as octo, then you can get to octapodi, octapodia.