I captured all of the monsters in the calm lands but one and then decided to go on to Mt. Gagazet. Then I fought the boss and was lucky enough to capture it.

After that I went back and caught the last monster in the calm lands but now I can't fight the boss in the arena. I think his name was defender X or something like that. But why can't I fight him?

I thought he was really hard, he took out all my summons right after I got off their overdrives (because that's all I could get off before he killed them in one turn). And he still wasn't dead even though they all did 9999 damage.

Will I not be able to fight him in the arena ever?

Defender X? The boss at the Gagazet entrance you mean? You're not supposed to capture bosses! Haha, it would have said "Defender X cannot be captured" if you beat it with the capture weapon as your last strike. So don't worry, you're not missing anything. But it's not just bosses that can't be captured, Mechs and Machina can't be either.
Basically, anything that doesn't turn into pyreflies, as well as anything that's missable will NOT be in the arena roster. You can encounter wild Defender X's later, but they're still mechs anyway. So... no need to waste effort trying to catch bosses, it doesn't do anything.

I wonder what monster you haven't caught yet? I wish I could remember what type it was, because I think I was in the same predicament when I was doing the hunts.

yes as said before you cant capture bosses or mechs. but somethin eles that was left out was was ghosts or "zombie monsters"

I just remembered that it is probably the Dark Flan in Zanarkand. It is listed for Mt Gagazet and yet it is over there. Yes I know, late reply.

Yeah, it could also be the tonberry in the cavern of the stolen fayth, they are really rare, i did like 80 fights before i came across one.

I just remembered that it is probably the Dark Flan in Zanarkand. It is listed for Mt Gagazet and yet it is over there. Yes I know, late reply.

There are Dark Flans in the Mt Gagazet caverns.

There are Dark Flans in the Mt Gagazet caverns.

Really? I didn't encounter one there.... Hmm, I got a lot of replaying to do *sigh*

the most pain monsters where that one bird near near the temple of jose.

tonberrys were also hard