Adamantoises sometimes show up, but mostly Fastitocalon-F’s. but don’t get discouraged. When you finally encounter an Adamantoise, do NOT mug it if you want an adamantine. They will only drop after the battle if the monster is not mugged.
I spent about an hour trying to figure out why I haven’t gotten any adamantine, keeping in mind that they were rare, then found out online they don’t drop if you mug them. 30+ Adamantoises will drop them every time. hope this helps
Personally, I go looking for them often. Great way to build up your APs very early on in the game (practically how I got Diabolo’s Enc-none very early on in the game).