What material was used to make the Trojan horse?

Pine, if you believe Virgil's Aenead.

By destiny compelled, and in despair,The Greeks grew weary of the tedious war,And by Minerva's aid a fabric reared,Which like a steed of monstrous height appeared:The sides were planked with pine; they feigned it madeFor their return, and this the vow they paid.Thus they pretend, but in the hollow sideSelected numbers of their soldiers hide:With inward arms the dire machine they load,And iron bowels stuff the dark abode.