Here is some information about how to determine lead leg.
  Last front leg on ground before air time.
Gait:
LH - left hind
RH - right hind
LF - left front
RF - right front

Below is the foot fall pattern for a race horse running at 55 fps.
Each foot stays on the ground about .1 sec with an air flight about
.1 sec. which is from the lead leg
There are three areas or times when two legs are usually on the ground
together (called overlap)
rear overlap - time when both hind legs are on the ground
front-rear overlap - time when first front foot hits ground and one
hind foot still on the ground
front overlap - time when both front feet are on the ground

  Horses use a diagonal gallop type of gait at 55 fps.

     Time Line: 0---------.1--------.2--------.3--------.4--

Stride time=.43 sec at 55 fps stride length=23.65 ft.

Right Lead:  LH  >---------                             LH  >---------
		     RH >---------
Diagonal Gallop>           LF  >---------         Air time
				    RF  >---------
					  Lead Leg
Rear overlap=..03 sec   Front-Rear Overlap=.03 sec  Front Overlap=.01
sec   Air Time=.10 sec

Horse will run turns on a left lead (US) then change to right lead for
straights to prevent fatigue.  That is why sprinters can go two turn
routes 1 1/16 and win and then lose at 7 f or 1 mi one turn races.

Large cats and dogs will used a rotary gallop.

Left  Lead:  LH  >--------                             LH  >---------
		    RH >---------
 Rotary Gallop>           RF  >---------
				   LF  >---------

Diagonal Gallop from rear to opposite front leg.

Rotary Gallop from rear to same side front leg.  ( Tiger in Exxon comm.)

Horse change lead during the air time when all four legs are off the
ground.  Jockeys can help a horse change leads in the stretch.