[Victariongreyjoy]

This is just my speculation of what forces Morgoth had in his first fortress before the Valar came and destroyed it. I'm just basing this off what we know from the Silmarillion.

Conventional forces:
-Early corrupted elves to orcs. We know he captured many elves during their awakening and corrupted them. I think these orcs were probably stronger than their late version, since these could have some powers from the Eldar.

-Ents turned into trolls.

Spiritual forces:

-Umaiar. I think he had up to 100 balrogs during the Year of the Trees. It was said after Utumno were destroyed, many of his creatures went into hiding. So that implies that dozen balrogs survived the onslaught of the Valar.

-Sauron.

-Pre-werewolves and vampires. I'm sure the forms of werewolves and vampires were taking shape already with evil maiar spirit in them.

-Formless demon spirits. Evil maiar that only were spiritual in nature. Maybe by forms of shadows as it was mentioned. These might be the opposite of fire-maiar, like the balrogs.

-Boldog orcs. Evil maiar taking shape as orcs. These were elite forces of the orcs, possibly huge and can inflict damage to the valar.

-Maiar trolls? I can see this happening.

-Spiders. Like werewolves and vampires, evil maiar taking form of a spider-like creature.

-Nameless things? There were probably unknown demon monsters that nobody saw except the Valar themselves. Most of these were probably destroyed and the few suvivors crawled deep into the earth.