got ambushed by one today near where you get a Mimeo Stone. SMN55 and was able to take it down, though a very close fight since it caught me off guard. i've never had anything in this game scare me so much...
I believe that many of the Antlions have the ability to burrow and ambush you. With Tec saying a Pit, and Ayus a Trench, and a Cave Antlion got me, it would seem as if there are a good chunck that love to ambush and take you down for a quick meal.
You're thinking of the Trench Antlion in the western portion of the map. One also caught me while exploring but I had Utsusemi up so I didn't get a gauge on the hit. Pit Antlion reside in the eastern side near the Tor. I was attacked by one but I took care of him with 70 hp left (57 THF w/o Utsusemi at the time) Was a very close fight.
You took one out as 57 THF/NIN O.o ...i cant believe that, or he was a very weak version of what i saw... it checked VT to me as a 73 NIN. I 2hrd as soon i checked, not worth the fight while Corse flys around me =/ Their hide abylity is a *****, when you see one, you are already agroed >_<
Pit Antlions are in the lower level section, and are about level 47-51. There are other types that ambush you over in the higher level section of the Chasm, which is what you saw.