From Late Latin troia, troja, of uncertain origin, but possibly:

from Latin (porcum de) Troia (“(hog of) Troy”), after the Trojan horse,
or alternatively of Celtic origin, from a Gaulish root *trogja, from a root meaning “to pull” and thence also “fertile," Proto-Celtic *trāglo-, for which see Latin traho.
or possibly simply of imitative origin, from the sound of a pig's grunt. Compare French truie, Catalan truja.