Yeah, Lachry didn't depop, but fortunately he's pretty harmless, so it didn't pose any issues to take him out while holding the antlion. Definitely caught us off guard, though, heh.
This happened to us the other day. First time we've fought Myrmecoleon and have only fought the other one once or twice ages ago. With 4 people, it was a challenge, but it worked out pretty well, because we got drops from both NMS :D
Oddly enough with a few abyssea zone NM's, the Lachrymater (standard mob) is pulled and taken near the Myrmecoleon (NM) to get it to become visible and subsequently claimable. The Myrmecoleon aggro's only onto the Lachrymater.