system
Given the following post, respond as if you're commenting on Reddit:
human
[40k] What did the various xeno-factions think of the Emperor of Mankind? What was going through their minds during the Great Crusade besides bolt shells and power swords?

Did the Eldar farseers have an understanding of the Emperor? Hard to imagine that they'd see just another Mon-Keigh, not when confronted with the Glorious Golden Glowing Guardian of Mankind.

Supposedly the Cabal were aware of the heresy and actively recruited for Horus under some guise of exterminating chaos, and John Grammticus said he met the Emperor on Terra, but why didn't the Farseers who ran the cabal tell the big E? Were they worried they wouldn't be believed and killed? Or did they know his plan was working and it meant their permanent loss of galactic boss titles?

Did the Greenskins have an opinion besides a source of good fighting?

I know the tyranids weren't around yet, and I don't think the necrons were waking up yet, but he did tango with the Void Dragon, so any c'tan who saw it probably has an opinion.

When he wasn't trapped on his god couch, was there anyone else in the galaxy besides the chaos gods who knew just how big and bad he really was?
gpt
We know the Eldar were split. Some supported the Cabal, some supported Eldrad. To his credit, Eldrad DID try to warn the Emperor of Horus' inevitable betrayal, but the Emperor was too busy building a piec of junk in his basement to listen. It seems that the Farseers knew exactly WHAT the Emperor was, and that was something of a problem... Because his very nature made coexistence unlikely. 

The Orks didn't really care. He was good for a scrap. 

The Necrond Triarch Praetorians were still active through the Great Sleep, and would have been aware of the Emperor... But not having an understanding of Psykers or Warp Dynamics, they probably just say him as a threat to Necron resurgence.