When I draw animals, I think it's helpful to follow the spine/tail as a basic guide, followed by the head and ribcage (and pelvis)... limbs and other features last. Sometimes the spine is in more of an S curve, sometimes it's more of a C curve. I wouldn't worry too much about following any strict rules. Focus instead on getting the gesture information down on the page in a direct manner. Break it down as needed. After all, what's an "S" curve", but two separate C curves combined? I hear you saying that it's hard to follow the spine but I do think the principle stays more or less the same between human and other animal anatomy. Keep in mind that not all photos will have a strong action to them. Some photos feel awkward or are a difficult angle and that's ok. Just do your best!
A couple of books helped me. For drawing animals specifically, I'm been loving the book "How to Draw Animals" by Jack Hamm. He will give you several blueprints to work from and show you how to block in the basic shapes. A more general figure drawing/ gesture approach that helped me was Force Drawing by Mike Mattesi. I think what that one helped with was gaining confidence in my mark making and also with how to start "feeling" the gesture as opposed to just drawing it. You're drawing a living breathing creature. Try to imagine bringing that to life on the page.
I'm sure others will chime in too... but honestly, a lot of it just comes down to practice. With time and effort, I think you'll start to get the feel of it. :)
[img]https://imgur.com/a/lIJg8B9[/img]
Gesture drawing Example
Finally, I made a quick example using some images I found of Toothless from How to Train your Dragon. Sometimes it's easier to see a gesture in someone else's art (note: these are not my images!). With Toothless's super long tail, though, I think it's easy to get a feel for the line of action. For these, what I wanted to do was to show you how I might approach these. The idea is to get as much information possible using just a few strokes. Again, I tend to start with the head or spine and kind of work from there.
I also stuck an example there of how you might get an 'S' curve using two 'C' curves. You can sort of "chain" them together as needed.
Hope that helps a bit!