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Stage 2: Draw the bones.
In the right viewport carefully begin to draw the bones using the line drawing as a guide template. To create the bones select Skeleton> from under Create> in either Model or Animate mode. Choose 2d chains as these are usually the most common bones in use. I began at the front right shoulder blade and draw a two bone chain. Left mouse click to begin drawing the bone chain and then middle click to end that chain. When you come to draw the front right leg it does not matter whether the bone ends match up as we will be dealing with that issue later on. The front right leg is two bone chain, so draw them using the real bones as an approximate guide. The feet are a single chain followed by a two bone chain and are very short. If you just want a rig for most walking, running and jumping actions, then this should work well.
To make the bones in the front leg into a complete fully functioning chain hierarchy they need to be aligned and parented together properly. We shall begin by using Transform> Match Translation to align the root of each chain to the end effector of the chain above it. This calls for careful selection of the correct nodes so I would use the schematic window to select those elements. This will ensure that separate chains move together when selected by the controls that we will eventually constraint to it. Open the schematic window (Press 9) and in it branch select the chain root of the the two bone chain and then in the Transform panel choose Match Translation and click the end effector of the bone chain above it. You will notice that it has aligned perfectly. Repeat this process until until the entire chain is complete.
Now we need to parent all these separate chains into a hierarchy by using the parent tool. Select the two bone chain's end effector and then select Parent and left click the chain root below and it will turn white as it is now parented to the bone above it. Right click to end this mode. Select parent again and repeat this process down the leg completing one section at a time and don't forget to end the parent mode after each section is attached. After that your schematic view should look like this schematic image.
You can now continue to create the back leg bones in the same way but they are less complicated. I created a single bone chain followed by a two bone chain. The constraining and parenting process is the same.(It is like the front leg but with no shoulder bone and it bends the other way.)
The chains we have just created are inverse kinematic chains and will require IK controls to be constrained to them. Before we can come to that we can draw some forward kinematic chains which will be the tail, neck, head and jaw bone. These will be animated by rotation only. You could decide to rig these as well for IK but FK will do just as well. These chains are created in exactly the same way using the image as a guide. Simply draw away from the body to create the tail, middle click and draw the neck, the final bone of which is the head bone. Underneath draw a two bone to be the jaw of the horse and parent this to the head bone. Seven bones should be enough for the tail.
