These are the first mobs you see when you enter Molten Core.
The packs of 5 corehounds near Lucifron and Magmadar was the first time we saw the "they all have to die at the same time" mechanic.
Usually the second boss fought in Molten Core, Magmadar required hunters to have Tranquilizing Shot -- the only use for this spell in the game at the time.
Garr was a challenge requiring many tanks and much warlock crowd control to handle his 8 adds. Garr randomly wiped out buffs and HoTs throughout the fight. He also dropped one half of the bindings required for the epic Thunderfury.
Geddon dropped one half of the bindings required for the legendary Thunderfury.
Sulfuron's adds all healed each other and Sulfuron. They each required a separate tank and had to each be killed out of range of the rest.
These sigils under the early MC bosses had to be extinguished before Ragnaros could be fought.
Executus could not be killed, but he surrendered and gave up his loot once you killed all eight of his adds. He dropped the items to begin the epic priest quest for Benediction, and the far more epic hunter quest for Rhok'delar.
The final boss of Molten Core, Ragnaros was a DPS check. He grew wroth with Executus for summoning him, saying it was "too soon" and then he smote Executus.