Awesome layered animation tutorial by Samy Fecih. Really well organized style of animating quadrupeds.
How to overlap a Tail - Brian Horgan
Slow Dog Walk Breakdown
How to light an animated shot. By Manar Al Tawam. Great stuff !
EMG-Zine - Gaits of the Quadruped
Oricom Technologies Walking Dogs
Fly Cycle Guide
Muybridge, Animals in Motion, 1899
Animation in Photoshop CC