Turning alphabet letters into animals is done by choosing the appropriate animal to represent each letter. The first letter of the animal's common name will be the basis for creating the alphabet. While some letters might only take a singular animal representation, others will require use of two or more represented animals to complete the letter. Abstracting the animals' shapes will be necessary to create discernible letters.
Keep the connection simple. Use the simplest and most common name of the particular animal species, such as dog and not German shepherd. For the letter "X," you will need to use an animal with that letter prominently in the name, such as ox or fox.
Abstract the basic forms of the animals, so they will fit the shape of the representative letter. This will allow for correct interpretation by the viewer. While an animal such as a snake representing the letter "S" requires little abstraction, other examples--such as a wolf for the letter "W"--take more work and the use of multiple animals.
Minimise your work by creating only half the letter. Many letters are symmetrical, either from top to bottom or left to right. Divide the letter in two equal halves; complete only one side. You then trace what you've just drawn onto the other side, completing the letter.
Identify the letter by enhancing it. Make the actual visual outline of the letter you are representing be very obvious to the viewer. Outlining the letter in a slightly thicker and darker line will ensure it is easily recognised in the animal shape.
Try to make your animal shapes as simple and straightforward as possible, in order to avoid visual confusion.