Юниты "Might & Magic Heroes VI"

Even the link provided by Drakkosha states that these birds are very rare and cannot be part of a multi-thousand-strong army.

So, multi-thousand-strong armies of Devils, Angels, Dragons, and Liches don't bother you?

Inconsistency #1. I was simply baffled that fire-breathing bulls were called "gorgons."

This is a complaint about the creators of the D&D bestiary; a similar creature seems to have appeared there. And these creatures are based on catoblepas.

Where did gnolls come from?

Study mythology; look for pesiglavs; that's what gnolls are (they were again called gnolls in D&D, and then it spread).

Also, the praying mantis from "Heroes 4" definitely cannot be a combat unit.

And how are they worse than, say, spiders?

I also haven't heard of any myths about "magog."

If you consider yourself a Christian, you should be ashamed. However, it's hard not to know about them anyway.

Djinn and fire djinn (ifrit). I don't see much difference. Then, gnomes and elves were put into one race, and almost identical unicorns and pegasi were added to it.

It's strange that you didn't mention dragons and golems, because they are much more similar than the creatures you listed.

Added 1 minute ago

And where did they come from there? And I remembered another unit: behemoth! Just a combat machine! Extremely epic!

Study mythology. Specifically, read about Leviathan and Behemoth.