Sometimes we are inspired by a book or a movie with a character or a type of character we see and say, "Great Googly Moogly! That would be awesome in my game!"
It's not always the easiest thing to do though. You have to find a way to insert it into your world and have a way to explain it. It gets harder if the one character is tied to others somehow. Then you have to dis-entangle it from the other characters as much as possible or just bring the whole bunch in.
Bringing the whole bunch in can be a bit easier for you, to a degree, but then your game can start to feel like it's becoming centered around the book or movie, etc.. that you pulled those characters from. You may not want that either.
For example, I was recently thinking about the Uruk-Hai from the Lord of The Rings.
I wanted to bring the Uruk-Hai in to my game because I think they were just freaking awesome in the movies and the books set them up pretty good too.
So, using some leads from the books and movies, I snipped them out of those and pasted them into my game.
For example, the Uruk-Hai are not "Natural" creatures in LoTR, they are created by some high power magic wielding person or maybe even demi-god.
In the case of LoTR, that would be either Saruman or Sauron. I don't want either Saruman or Sauron in my game though.
Also in LoTR, the Uruk-Hai are related to Orcs. I don't like that. To me, Uruk-Hai are related to Orcs the same way a lion is related to a gazelle. The Orcs and gazelle are something to be eaten, not much more to it.
So, I changed the Uruk-Hai's name. Yup, to Uruk'hai. I know, amazingly creative of me, but in my mind at least, there is a distinction.
Uruk'hai still only exist by mystical/magical creation though. They aren't "naturally occurring" like Orcs or humans, elves, etc...
So now I had to figure out, what whackjob of a person is going to be able to make these things?
I tossed the idea around a live chat with a fellow DM and between us, we determined that it is a cleric, an insanely evil cleric, who is capable of raising the Uruk'hai. Yet, how does even a high level cleric able to just pop these things up. He's gonna need some high powered help here and like I said earlier, I don't want to bring in Saruman or Sauron.
What if, I did use Sauron as an influence? Not create a demi-god or god named Sauron, but a nutball, power hungry demon god that was bent on world domination like the "old ones" in an H.P. Lovecraft story? That could work.
So now, I have a demon-god bent on world domination that will help and direct a special, nutjob evil cleric who follows him to make a bad ass race of beings that are born to be a hit squad. What if the newly raised beings have it built into them to be followers of this demon god, whose name is Uruk-Khan and his warriors are called the Uruk'hai? Oh yeah. I likes this.
Uruk-Khan can give the cleric the ability to raise and control the Uruk'hai as long as they are doing a good job of destroying and dominating everything they see. The minute that Uruk-Khan decides that the whackjob cleric isn't getting the job done or isn't playing the game his way, he will remove the ability to control the Uruk'hai from the cleric and let the Uruk'hai run free in the world.
Now we have a way to control the Uruk'hai because on their own, they aren't afraid of anything except Uruk-Khan. Even if they confront something bigger and badder and they know it, they'll go into berserker mode and die trying to bring it down anyway. That's what they were made to do.
Now I get to have Uruk'hai in my game and a new demon god to motivate an evil nutball cleric and I don't have to have the others dragged into my game at the same time.
Believe it or no, I already have a cleric that fits the bill in my game too. This is going to be so much fun.