When you become a werewolf, where does your follower go? Cant find him either where i saw him last (outside the underforge) or where he was originally (the inn at Solitude). How to find him?

they have default locations, not always where you first run into em..like jenna goes back to the lodge in whiterun..the college mages go back to the college, they go back to where you first got em...but sometimes they will go to another spot, like erandur or eola, they go to their new locations after you help em with quests.

The followers have quests?
Do you know where i can find the follower i lost? Dont remember his name, but you can hire him for 500gold at the inn in Solitude. He is kinda a warrior mage.

I believe in general most followers will return to their original place if you release them from service. If you've told them to wait somewhere but you don't remember where you'll need to let a few days pass (3 I believe) and they will automatically return. So, if you can't find the guy use the "wait" command or just play the game and let three in game days pass and you should see a message about a follower leaving your service, your guy should be back in Solitude after that.

One other thing to try, if you think your follower may have accidentally died, is to try and get a new follower to join you. If they tell you that you already have a follower you know he's still alive somewhere.

He should be back at that inn. Granted, the only time I hired a merc, I parted ways with him in the inn (in Markarth) where I hired him. It's possible that your merc met a bad end on the way from Whiterun to Solitude, or simply hasn't made it there yet.