Sculpting figures of animals can be a fun and rewarding hobby. A material that many artists use for this task is called polymer clay. It will not dry out due to contact with air, and can be baked hard in a household oven. You will sculpt your cow on an armature, which is an under-structure that gives support and fills in bulk, reducing the amount of clay that you have to use.
Cut two pieces of stiff wire, each long enough to serve as two legs. Cut a third piece to be the tail, spine, neck and head.
Wrap the first leg wire around the spine where the rear legs will be positioned, forming a cross shape.
Wrap the second leg wire around the spine where the front legs will be positioned.
Bend and adjust the legs, tail, neck and head portions of the wire into the position that you want for them. Use images of actual cows as a guide if desired.
Add bulk to the torso and head by packing on wadded aluminium foil.
Brush the armature with polyvinyl glue to help ensure a good bond to the clay. Allow this glue to dry before sculpting.
Add polymer clay over the entire armature, keeping it 1/4 inch thick.
Smooth the clay with your fingers to remove any noticeable lumps or problem areas.
Add details to areas such as the cow's hooves, udders and face using sculpting tools. If you are unhappy with a detail, wipe it smooth and try again.
Set the finished cow on a baking tray once you are done sculpting.
Bake the cow according to the instructions on the polymer clay packaging.
Remove the cow from the oven once it is done baking and allow it to cool.
Paint the cow with acrylic paints as desired.
Do not try to sculpt a solid cow. If polymer clay is too thick, it may explode in the oven.