You don't need to be the world's most artistic person to help your child create his own horse mask -- there are plenty of resources out there to help you. If you really can't face all the glue and paint, buying a mask premade or partially-made is also an option.
Making a Horse Mask With Paper and Paint
Draw a horse's face on thick paper or card stock that is about the same size as the child's face. If you're not sure how to draw a horse's face, make the shape similar to that of a fat figure 8, making sure to draw ears at the top. Use a picture of a horse to help. If you cannot draw it yourself, find a picture on the Internet, print it out and stick it to your card. Hold the mask in front of the child's face and mark where to cut eye holes. Set the mask down and make the holes. Check again for correct size and positioning. Proper placement of eye holes is important because the child needs to be able to see for safety. Cover your work surface and put an apron on your child if she is painting.
Paint or colour the mask, then leave it to dry. Make holes on both sides of the mask, not too close to the edge. Reinforce these holes using sticky tape. Thread elastic through the holes and measure to the size of the child's head before cutting and securing with knots.
Making a Foam Horse Mask
To make a soft foam mask, buy sheets of soft foam in different horsey colours from any arts and crafts supplier. Draw the mask shape with ears on the foam, making sure it is roughly the same size as the child's face. A good shape for this mask is a fat figure 8 or a bone shape with triangles jutting out at the top for ears. Use sharp scissors to cut it out. Hold the shape in front of the child's face and mark where the eye holes need to be. Take the mask away to cut the holes and then check again with the child. It is important to make sure the child can see easily through the holes when wearing the mask.
Cut out extra ear shapes, slightly smaller than those on the main mask and in a different colour, and glue these on the mask ears. To add more detail, cut out a tuft of hair in foam for the top of the mask and a thinner bone shape to sit between the eyes and down to the horse's nose. Glue these on and leave them to dry.
Make a mark for holes on both sides of the mask, about half way up and not too close to the edge -- these will be used for the head elastic. Put a blob of plasticine or similar soft material under the mask before punching through the hole with a sharp pencil. Reinforce these holes by adding sticky tape and punching through again. Tie elastic through one hole, then measure the child's head to get the right length before cutting and knotting the other end.
Making a Native American Horse Mask
To make a Native American horse mask you need a piece of thick paper or card stock roughly twice the size of the child's face and natural materials to decorate. Feathers, small beads and brightly coloured paper are ideal.
Cut off the corners of your card in a rounded shape to give you the mask shape. The mask should be wider than the child's face so it bends in a curve when worn. Hold the shape to the child's face and bend around their face. Mark large eye holes close to the top and centre and remove before cutting out. Check again to make sure the holes are in the right place as it is essential for the child to be able to see.
Cut out pieces of coloured card or material to decorate the mask. Square and triangular shapes work well to create Native American style patterns. Stick beads and feathers around the mask and leave to dry. For more detail thread beads onto some string or attach feathers to string and hang around the edges of the mask. You can attach these with sticky tape or by making small holes, threading the string through and knotting.
Make holes on both sides of the mask, not too close to the edge. Reinforce these holes using sticky tape. Thread elastic through the holes and measure to the size of the child's head before cutting and securing with knots. The headband should help to bend the mask around the child's face.
Making a 3D Horse Mask
If your child prefers not to wear a mask on their face, you can make a 3D Native American horse mask to be worn on the head.
Measure a length of card, roughly 3 inches wide, to fit around the child's head. Join the two ends with sticky tape to make a headband. Take a second piece of card, roughly 8 inches by 12 inches, and decorate in Native American style. You can colour triangular patterns on the paper and stick feathers on to create the look you want. If you have glued, wait for this to dry before rolling the card into a cone shape, roughly 2 inches in diameter at one end and 4 inches at the other. Attach the narrow end to your headband using sticky tape.