Fairies:

Little winged fellows, cavorting in the meadow, kissing dew drops and granting wishes. Oh, what if you were real? Well, first of all, we'd have to develop some kind of faericide to kill them off because some people just wouldn't stand for that cavorting crap. So, we'd probably have to make new laws clarifying their subhuman status because, hey, they're gonna get killed a lot. A baby's like "Oh goo, what's that?" Smoosh. Then the entomologists would ether them and pin them up on corkboard with labels and sell the fairy dust they scrape off their abdomens to pharmaceutical companies. It'd be valuable stuff too, because fairies would only last a few generations what with their inability to reproduce… y'know, because they're all gay.

Witchcraft:
No, it wouldn't be like Harry Potter, you fairy. It wouldn't be special and it wouldn't be fun. It'd be like science class. They'd call it something stupid like extrapredictive arts instead of magic and you'd get school-issue wands that look like rectal thermometers. There wouldn't be any magic words. That's dumb. It'd be a bunch of memorizing diagrams and angles and you wait through six weeks of intro before you get to turn sand into salt or something equally lame. Witchcraft majors would spend most of their time trying to rationalize their trade in the face of computers or trying to make potions that get you really high without making you impotent or purple.

Giants:
Oh man, they'd all die so horribly. Big and hunched and naked. What could clothe them? They'd spend their short lives wandering around, all blind and ignorant, just pissing on everything and destroying stuff. Most would die either of starvation or they'd suffocate due to the lack of oxygen in the upper atmosphere. The National Guard would take out the rest — why not? Bush executed the handicapped — or maybe they'd just send them to foreign nations to have a "tumbling contest." In the end, they'd just leave behind giant bones, mayhem, and a bunch of women with, like, way, like wow stretched out vaginas.

Unicorns:
Well… it'd be like how horses are now… but some would have horns… Maybe unicorns would be more expensive than horses? I don't know. Seriously, whoever came up with unicorns was really phoning it in. "Hey how 'bout a pony with a thing in the middle?" Retard.

Mermaids:
Reality check, they wouldn't be hot jailbait redheads. No way. They'd be all blue and cold and scaly with nappy-ass dreads for hair. They'd have a fish mouth with all these little teeth inside and their hands would be like claws so basically they'd totally suck for giving head. What? Well, what else would they be good for? No pussy — fish tail. OK, they'd have boobs. Only fair. And none of this seashell bra crap either. Think more like Mystique in the X-Men movies. Y'know, good enough. Good enough for an illicit rendezvous down by the docks and then you knife her in the gills so no one can ever know that you like it wet. Also, they'd make neat jewelry out of shells.

Leprechauns:
Hello? They are real.