Santa Claus: A joyous childlike old man but I imagined him fatter...
Easter Bunny: I imagined him small and always carrying a basket of eggs.
Sandman: I didn't know about him until I was around sixteen.
Tooth Fairy: I imagined her more motherly and looking like a fairy not a hummingbird.
Boogeyman: I thought he was covered in boogers and hid in closets.
Jack Frost: Thought he was a villain who only tried to hurt Santa.

I always thought he looked like a Golden Pitch... But the personality was spot-on!

I was told he never talked because he would awaken the children...

Jack Frost was always thought to be bad. I heard about him in the Rainbow Magic series where he has a beard, a wand and goblin servants.

rainbow magic makes everything seem different

I know right?

heh

I never really had a clear image of any of them as a child; they were always shifting based on new things I learned about them, and I never thought any of them were real since I was taught that they were just embodiments of holiday/whatever spirit. I guess Sandy has the most elements I considered, though.

Tooth is easily the furthest from what I thought, since we had a tooth FERRET in our house, and I've never associated any kind of tooth fairy with hummingbirds. I don't like how it came out, either.

In all honesty I didn't exactly believe in the Sandman when I was a child, Santa don't really look like Santa to be, the Easter Bunny or Bunnymund looks more like that rabbit from Wallace and Gromit, the tooth fairy is NOTHING like I though she's look like, I didn't exactly know about Pitch and I always imagined Jack Frost to have white hair and be a teenager....I don't know, don't hate me!

Here's how I saw the Guardians when I was younger:
Santa: Big and fat
Easter Bunny: Big, white and cute
Jack Frost: Who?
Tooth Fairy: Multicolored fairy
Sandman: Some tall golden man
Boogeyman: Sexy, tall and scary animal or human

I always imagined Santa being fierce

Most were a surprise (I didn't know Sandy, Pitch, and Jack back then). Bunny and Tooth were far from what I expected, and I honestly didn't think much about what they looked like. Santa though, apart from the tattoos and sword skills, my image of him wasn't really far-fetched.