The possessive form for porcupines would be porcupine's. Since a possessive noun is a noun that shows ownership, there should always be an apostrophe s ('s) in any words such as porcupine.
It depends on whether you want the possessive form of porcupine (singular) or porcupines (plural) because there is a difference. 1. singular porcupine: just add an apostrophe followed by -s: porcupine's 2. plural porcupines: just add an apostrophe (not followed by -s): porcupines'
