Gaint monster? Allied seals?

[Broken_Arrow]

So I was doing a quest and there was a giant monster a bunch of people were attacking. It was level 50 and I was level 24. I started attacking knowing it probably wouldn't aggro me. After it died I got some allied seals. . .what are they for? How/when do these monsters appear? How often do they appear? Is there a listing of them?

[Eye_Gore]

Those are the hunt marks. When you reach level 50 and max GC rank you get a quest "Let the Hunt Begin" Which unlocks the hunt board. You have dailies and weekly marks to collect Allied Seals you can turn in for gear, materials, housing items, etc.. The S and A ranks will also award Tomes for exchange in Mor Dhona for the same kind of stuff. You can, as you saw participate in those. The others you need to unlock down the road.

The A and S ranks like you saw require many people to defeat. The dailies are just regular monsters that you kill multiple 3 or 4 of. The weekly mark is solo-able but for a fresh 50 could prove difficult.

[Niwashi]

You encountered either an A Rank or S Rank Hunt mark. Anyone can fight them. (It will take quite a few to succeed - especially for the S Ranks.) And for successfully killing one you can get Soldiery Tomestones and Allied Seals, both of which are currencies for iLevel 100 endgame gear, and in the case of S Ranks a few Poetics Tomestones as well, for iLevel 120 gear. The amount you get is dependent on how strong your contribution to the kill was, so at level 24, you likely got only a portion of what a group of endgame players would. The max rewards are:

A Rank: 30 Soldiery, 40 Allied Seals
S Rank: 100 Soldiery, 10 Poetics, 100 Allied Seals

To trade Allied Seals in for gear, there's a Hunt Billmaster at your GC headquarters who will trade them. And to trade in Tomestones, there's a vendor named Auriana in Mor Dhona you can trade them to.

Due to the difficulty of gaining a full contribution score when doing this at low levels, plus the fact that you can't use the gear until level 50 anyway, I don't recommend actively searching for these mobs until you get closer to endgame (maybe mid to late 40's). Of course, if one appears near where you're at anyway, like on this occasion, go ahead and join in. Just wait for enough other players to show up and once they begin the fight you can attack as well.

Here's a listing of the Hunts available:
http://ffxivhunt.com/malboro