HIVE

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Sums it up nicely, I think.

And what happened to the Human's Pegasus knights?

I think they are playing dodgeball with golems.
It's a fun sport for everyone.
Anyway you can see them in chapter 4 of the second book so they are still there. I don't think we will get them as trainable units however...

Then the Humans are the only race in the books that doesn't have any trainable flying units.

Don't forget destructors.

That's a Siege weapon...
And it's only available on the SHB.

I don't see a need for additional air units in First Human Book.