Depending on the kind of plaster you are using you will either have a pliable mass of plaster or a pourable liquid. If using modeling plaster you can form it to your whims but liquid plaster is a bit harder to deal with. For pourable plaster you need to have a mold ready made. These can be purchased or made by hand. To make a mold you will need to build a mask, exactly the way you want the plaster one to look out of a material such as paper mache. You then wrap a mold-making solution around the face that you can press the mask into until it has hardened completely before breaking out the paper mask. Clean out the mold of any dust, then pour in your plaster and wait for it to dry before removing.
Animal masks come in a variety of shape and sizes and use many different supplies to achieve the desired effect. Embellish a poured plaster mask using feathers to create a bird mask. Or you can make an elephant mask out of modeling plaster, which will allow you to sculpt in detail. An interesting alternative would be to make an insect mask using bits of shiny paper (or recycled product wrappers) to make a collage-like effect for eyes.
Masks vary hugely depending on the culture. Some Native American tribes make masks out of animal heads or fur, which you can imitate by covering a plaster mask with cheap imitation fur. The Chinese create expressive opera masks with strong facial features but few small details that are well suited to poured plaster projects. Vejigante masks, popular in the Caribbean and in Spain, are large masks painted in bright happy colors and a variety of textures. They also feature large horns, spikes and scary teeth that demand some degree of sculpting or modeling to construct.
Fantasy masks do not have to follow any rules of form or construction and can look any way you want. You can make a unicorn mask --a horse head mask with a horn, made out of modeling plaster-- or an elongated alien head mask. Drill holes in the mask while it is still damp and make piercings or string beads or even nails through it. You can glue a wig onto the mask or give it facial hair such as a neon pink goatee.