Another option for an interior armature could be styrofoam packing material.  Not the 'peanut', but the bulky forms that protect things like electronic equipment.  They are often kind of irregular in shape, but can be broken, or cut with a serrated knife.  Just glue them together. 

For your bull, a basic whole-body armature of 1x2" wooden sticks should work well, then you would apply your styrofoam chunks onto that.  DON'T EVEN TRY to make a PM/etc body and then attach legs to it afterward.  It won't work, it will fall apart, and you will have wasted a lot of time and energy.  Make kind of a basic skeleton as in the '...cockeyed...satan...' link above --- support for the body, head/neck and legs.  It all has to be joined firmly together internally, or it will fall apart.

I usually use foam panelling glue in the cardboard cylinder (requires a cheap dispensing 'gun').  It isn't perfect, since it takes at least 12 hrs to set, but it doesn't melt the foam, which many adhesives do.  I use spool wire and/or cheap bamboo food skewers to pin the pieces together while the glue is drying (and often leave them there, unless there's a reason to remove them). 

Low-temp heat-melted glues work great, but they can be kind of expensive for a large project. 

Once you've got your basic body form, you can fill any gaps by gluing small chunks of styrofoam in them, or stuffing them with wads of newspaper.

Then start layering your papier mache strips.

After it is thoroughly dry, you can paint it.  See Jackie's piece under either Tutorials or Articles above.  She did some experiments and found marine varnish to be the most weather-resistant.  Not regular varnish, which won't stand up to weather.  And you'll have to coat it very well, not leaving any small holes or cracks unprotected, or the bull will absorb moisture, and you'll have a sorry-looking mess on your hands.  I would try to keep it protected from the weather as much as you can. 

Sue