The sequence of footfalls is the order in which a horse's feet move at different paces. All paces are shown in this post.
Walk = 4 beats
(1) Left hind (2) Left fore (3) Right hind (4) Right fore
Trot = 2 beats
(1) Left fore and right hind together (2) Right fore and left hind together
Canter (Left foreleg leading) = 3
(1) Right hind (2) Left hind and right fore together (3) Left fore
Canter (Right foreleg leading) = 3
(1) Left hind (2) Right hind and left fore together (3) Right fore
Gallop (Left foreleg leading) = 4
(1) Right hind (2) Left hind (3) Right fore (4) Left fore
Gallop (Right foreleg leading) = 4
(1) Left hind (2) Right hind (3) Left fore (4) Right fore
Walk = Tracking up
Trot = Over tracking
Canter = Under tracking