Video: http://www.youtube.com/watch?v=6_mIQaTG4BE

Me and my husband made the hooves together. They are 4" heel boots with the heels ripped off and PVC pipe stilts attached to the bottom of the toe. A metal piece stabilizes the shoe above the pipe and cardboard forms the shape of the hoof itself. They made me 14 inches taller.

I could walk in them just fine, but they were not for walking or standing in for long periods. They took some leg muscle to maintain balance and the pressure hurt the balls of my feet after a while. I was also sensitive to uneven paving and deep carpet. Ramps are a no-go. I had a secondary 7" pair for walking distances.

I also made the horns, ear, tenticles, tail, armor, fangs.

I have started a collection of hoof construction photos and info here: http://www.cosplay.com/gallery/162790/
My construction technique is ideal for dramatically increased height, exposed calf and thigh silhouette, and slender, disproportionately long legs. They are also fairly upright, with the stilts coming close to being perpendicular to the ground. If this doesn't fit your costume, I would recommend looking at some other options:

http://www.transform.to/~mwbard/stuff/thehorsetailor/indexcomfyhooves.htm
Good for bulky digitigrade legs that are proportional in length to the rest of the body. Most comfortable option. Could be used with a heeled shoe or flat stilt (ideal for Tauren) to be taller.

http://gryphern.googlepages.com/
Good for slightly bulky, more angular digitigrade legs. Intermediate height increase. Requires braces on the legs. Probably better than mine for people unaccustomed to walking in heels.

The horns are sculpted out of foam. It was a very long process, so I would recommend paper clay instead. I attached my horns to my wig with flat-ended sewing pins, so that the flat ends were against my head.

The tail is made out of a foam pipe cover from the hardware store. They look kinda like pool noodles. I found mine in the pvc isle. I cut long, rectangular pieces out of it to make it taper to a point, then glued and taped it back together and covered it with fabric. I'll try to remember to post a photo. The tail is attached to the belt I'm wearing.

When you spray paint spandex, then pull on it, it naturally rolls. I noticed a scrap do this and used it for the tentacles. I just rolled the fabric, lightly glued the overlapping edge, then sewed them to the costume. My costume has a hood of purple "skin" that goes up over my head and functions as a wig cap. That's what I sewed them too. But you could also probably sew them to the wig.

I bought the bodysuit on eBay. It was gray. I lightly spray-painted it. I really like the painted look for bodysuits because it no longer has the characteristic texture and shine of spandex. The downside to painting, is that it becomes less breathable and it is difficult to paint the seams. I find that stretching the seams over my hands works okay for getting the paint in there. There are ways to dye bodysuits as well.

Anyway, I bought a suit with an attached head, cut out the face, and used the head as a connected wig cap. It isn't as simple as it sounds. It required alterations around the neck and waist the make it fit right. I also had to precisely match the face paint to the suit.

The face paint I used was Ben Nye cream. It is great because it is opaque and comes in bright colors. However, it is also shiny and looks bad in flash photography unless you apply a translucent powder over it. Translucent powder application is something that I personally always fail at. If you use a regular foundation under the Ben Nye, it will stay on all day.

All jewelry is made out of super sculpey.

I think this guy is using the same technique for the low hooves. You can see how easy they are to move in: http://www.youtube.com/watch?v=9f5Y-OqyL4g