Here's a walk cycle of a horse, animated from four angles by Simon Otto in preparation for Spirit: Stallion of the Cimarron. (link to YouTube)
Whether you're a painter, sketch artist, or an animator, it's helpful to study frame-by-frame breakdowns of walk cycles. Still one of the best sources is the pioneering work of Edweard Muybridge, who took photos of horses in motion as early as 1878.
|Muybridge horse walking with rider|
Here's one basic observation about a walking quadruped. Typically when one of the front legs passes the other's position, the back legs will be at full extension (first and last frame above).
Similarly, when the back legs pass each other (frames 6 and 7), the front legs are at full extension. (Note: read the action from right to left starting in the second row.)
Even if you're a painter and not an animator, it's good practice to sketch a few key poses so that you can generate them from memory. This can be a big help later when you're sketching living animals. When they're moving at normal speed it's almost too fast to observe.
Here are some resources if you want to explore this topic further.
More about Simon Otto