A unicorn costume can, of course, be constructed for two people much like a pantomime horse [see Wikipedia]. However it would take quite a lot of dexterity to ski in this configuration! There are a lot of commercially available panto horse costumes that you could easily turn into unicorns! However they do tend to be a bit on the expensive side. Or you can go for a basic pre-made model for a few hundred dollars.
FULL BODY COSTUMES
The Aeromax toddler unicorn costume is an easy zip up design which makes it a very simple costume option although the pink color will probably limit its appeal to girls.
Adult versions of the costume are typically home made. These suit work best when modeled on an anthropomorphic (a.k.a. "furry") vision of the unicorn. This type of suit strikes a harmonious compromise between equine and human attributes. However the investment required to get this type of result limits its use mainly to true enthusiasts of 'fursuits'.
And for a slight change of pace, see this unicorn zentai ("complete body suit, 2006).
Other unicorn fursuits:
Four-legged costumes (centaur)
There are a number of ways to achieve an authentically four-legged unicorn costume.
Firstly, the rear part of the body can be constructed and attached at the waist to be worn in an upright position. The rear legs can be stationary, lightweight or mounted on wheels.
Four-legged costumes (arm stilts)
Alternatvely, a quad suit can be constricted wear the front legs are operated using arm-stilts. See an excellent example here.
HEAD ONLY COSTUMES
Horse-face masks: Pictured right is an authentic Venetian unicorn mask which maintain a horse shaped face with eye-holes near the unicorn's eyes.
There is a widely available rubber unicorn full-head mask that is very effective (see below). It is a version of the horse head mask made by Archie McPhee and Company. This developed into the brief 2013 fad of unicorning--wearing the uncoirn mask in baffling and inappropropriate settings like supermarket shopping or site-seeing.
Probably the definitive cheesecake unicorn is the young man in a well known this Exterface photoset. These photos even have their own fan art.
See also this interesting club costume.
Costumes for Pets
For a few more costume check out my hub on pegasus costumes. Costumes marked with an asterisk (*) also have a unicorn horn.