Xombie Master said:
This DM didn't "realize" they were monstrous humanoids until last night.
See, this is the point where a good DM will consider a retcon.
"Hold on," he should say, "I just noticed these things are Monstrous Humanoids."
"What?!" The ranger says. "My character has lived in this world for thirty years, and knows a lot about giants. These things are common. Did
I know that they were Monstrous Humanoids, or at least that they're not giants?"
"Um, make a Knowledge: Nature check. DC 10, since these things are common, and you can add your favored enemy bonus onto the check. Anyone else in the party can make the same check."
"Got a 13 on it. So I knew that they were monstrous humanoids, way back when? Can I just switch those giant-bane arrows to monstrous-humanoid-bane arrows?"
"Sure, go ahead: that's what you would've bought if I'd been paying attention anyway. Sorry about that!"
And the game goes forward.
Note that the DM can also try to incorporate the screwup into the game: perhaps normally war-trolls in his campaign ARE just regular trolls with fighter levels, but these were a whole new creature who'd been trained to act like trolls, for some who-knows-why reason that leads the PCs down a new path. However, you have to be a very adroit DM to pull this off, and even if you are, you still risk irritating the players. Better IMO to do a simple retcon like the one above.
Daniel