Golly!  That's a BIG question but I'm sure you'll get some responses and a variety of ideas on this site.  Since Jackie and Russ set it up there has been the best flow of papiermache examples and ideas anywhere in the world.
The armature is the first issue.  It has to have strength as well as providing the fundamental shape.  Is this to stand outside or in a building?  Remember that pm is never absolutely waterproof, however hard you try.
If you are geometrically minded, a structure of interlocking pieces of cardboard would be very strong and would shape well.  You could form the body and then add legs and head as separate units.  Pad the interstices with screwed up paper and apply about ten layers of strong paper with paste all over.    I would then apply a fine pulp for moulding and smoothing.
Personally I haven't used plaster over pm but would doubt it.
If the cardboard idea interests you, just say and we suggest more about ways of doing it.
I now use a lot of foamed insulation board (as used on building sites) and it is possible to construct large scale with this and model it to quite reasonable detail.
If this interests you, just say and we can suggest more about ways of doing it.
DavidO

Here is a site that shows the interlocking cardboard: http://www.cs.cmu.edu/~rapidproto/stude … /project2/

However, you don't really need it to interlock, just make it look that way. You can cut identical pieces and hot-glue them onto a base. You could cut out the silhouette of the horse, then add matching pieces to form the bulk of his body. But you would need to anchor the entire 'skeleton' to a base. The three hooves down with one raised does form tripodal base, but it wouldn't be steady enough by itself.

David's idea of sheet insulation would give more bulk more quickly, but cardboard is cheaper and more easily available. But a combination could work very well.

Once your form is absolutely as you want it, including the smoothness of the PM, I "think" you could use plaster over it.  To make sure, make a smallish non-complicated object with a good PM surface, let it dry thoroughly, then apply plaster to it. In fact, use this object to test all materials you're planning on using on your horse. Make your mistakes on it, not the horse!

I would make very small batches of plaster, then apply it quickly with a paintbrush (1 to 1.5"/2.5-4cm wide). Just keep in mind that the amount of plaster on a lightweight object three feet tall is going to affect the weight and balance of the object, so be sure to plan ahead. And a solid multi-layer PM surface would be required. No thin, lightweight skins would do.

Tips for plaster:
Warm water for plaster makes it set up faster, cold water is slower.

Always add plaster to water, not water to plaster.

The setting and curing of plaster is a chemical action, not a drying situation.

Don't let plaster bulk up on your hands, it can cause horrible burns. Use latex or nitrile gloves. This should not be an issue, but some people don't know it.

For safety, use an N-95 paper dust mask (any paint dept. or store) when sanding plaster -- once it's in your lungs, it's there forever.

Sue