1) How much do you either A) Create your own monsters or B)Incorporate monsters into your game that are not from the system you're playing?
It depends. Mostly, I use things as written, but I'd guess that at least 1/3 of what my players face is tweaked, altered, lifted from other games/genres or made up by yours truly.
And that's pretty much across all systems. I have a War of the Worlds type campaign in which the aliens were Mind Flayers...and another one I ran in another city in which the aliens were Beholders. The Beholders were a bit watered down, though, and had fewer "powers".
Over time, in various campaigns I've done things like:2) What are they/have they been?...and Where did you pull them from? (obviously)
I made a race of minor- but numerous- demons called The Dark for the main foe in a campaign once, but I screwed up in their design. I had Dwarves who were living stone, and Elves who were alien "Greys."
I tweaked Warforged to be more like Cybermen (with Dwarven brains & culture), and had Elves that were plantlike Fey.
I've also lifted the Seshayans from Alternity/D20 Modern, Wolfen from RIFTS, Nehwon Ghouls from Fritz Lieber.
One campaign had standard PHB races get trapped in a world where all the major races were Anthropomorphs.
Outside of D&D, I've done things like have Martians who were affected by opium like a combination of PCP and zombie powder (HERO rules, Space:1889-based setting).
3)How did it go over with your players?
The Dark were fatally flawed, but nobody noticed anything except how easy they became to beat over time. So it was kind of an anticlimax when they were ultimately defeated.
Others have been moderately well received. But a lot of my current group like their D&D to be plain vanilla.
The Mind Flayers/Beholders as Martians in War of the Worlds campaigns went over pretty well.