Achatina's are by far the hardest animals to escape on the island. Their insane movement speed is devastating to you and your dinos. These snails are also very powerful and will kill your gigas within, at least, 24 hours! The best thing to do when you see them is bola them and run.
You may find the legendary Gary by himself or with his mates while running along the beach. You can find him at South, West,and East Zones along the shores. If you encounter him tell him his homie Merk sent yall. ~Tundra