There are 3 types of mini-bosses in Team Arena - Giant Komodo, Golden Spider, and Thoracotomy. These mini-bosses are hostile to everyone (i.e. both teams). When they spawn, there will be a yellow notice on the top of the screen:
Thoracotomy/Giant Komodo/Golden Spider monster has appeared.
This means that the mini-boss has spawned in one of the 11 spawn spots. Several spots are in the patrol route of the mobs, so they will come under attack by the mobs and sometimes by Scylla and King of Greed.
These mini-bosses are higher levels (about Lv.125-135) than their normal counterparts, but this can easily be compensated by Team Arena buffs. They can drop the same items (e.g. Lv.92 Enchantment Chip, Lv.84 Elite Equipment). If most of the damage is done by players, they also drop about 50-70 Arena Coins each. They don't seem to despawn after Dios is killed, so it is possible to encounter them when opening the treasure chests at F6/G6.
Unlike the Kings of Greed in Team Arena, these mini-bosses inside Team Arena count towards the kills for boss hunt missions. These missions can be acquired at Rescue Knight Militia in Ustiur Base Camp and at the colony managers of Rio Albi and Lago Celeste. A completed mission gives Lv.80 Polish, Amber, and 30 reputation points. So if you do Team Arena often, this may be an easy way to acquire more reputation points, which are needed for the instanced mission raids.
For other issues relating to Team Arena, see [v3.2 Feature] Survive the Team Arena! and Team Arena FAQ.