This area has an optional summon and can only be accessible via Submarine.
Head to the northwest continent, then enter the Cave.
Cave
This cave has one floor. Get the two bestiary entries, then exit to the Overworld.
Bestiary Checklist:
Chocobo Forest
This enclosed forest has the summon call Catoblepas. Just like Ramuh, it's a random encounter, so you have to run around a bit. Once you get the summon, enter the Chocobo Forest to open a new area for achievement.
Nothing is in here, though you can try catching the Chocobo for a one-line dialogue.
Boss: Catoblepas
Guaranteed Drop: Catoblepas
The boss will counter both physical and magical attacks with Demon Eye, which will petrify the damage dealer.
So you need to prepare a lot of Gold Needle or use Esuna if you plan to kill it the normal way.
There are 2 best strategies to deal with Catoblepas, both with pros and cons.
Strategy 1:
Set Mix ability on everyone, then keep spamming Mix: Maiden's Kiss + Turtle Shell.
This is the easiest strategy but requires about 5 to 6 Maiden's Kiss and Turtle Shell.
You can farm Turtle Shell here from Land Turtle.
Strategy 2:
Requires a bit of preparation. You need to catch an Ironback from the Cave by using Death Claw (Blue Mage) then Catch (Beastmaster).
During the battle with Catoblepas, Release the Ironback to deal about 4000 damage.
Death Claw tends to miss a lot too.
IMPORTANT: you receive Catoblepas as an item, so open your Inventory -> Items, then use Catoblepas to add it to the summon list.
Backtrack to your Submarine.