There are two methods. You can either:
Note that both of these methods won't work 100% of the time; there is only a small chance of getting a Harvest Trevenant in a Horde, and there is also a small chance of getting a Harvest Phantump in a Friend Safari when that friend is online.
You can get trenevant with the harvest ability in a horde battle with trenevants.
You get it by luck in a horde though. You don't get it on the first try.