Thanks to Dave Bird---St Hippo of Augustine for the description of the gaits:
- Slow Walk: The legs act in diagonally opposite pairs: 1. LeftRear forward 2 RightFront 3 RightRear 4 RightFront, in four time units the animal is one pace forward.
- Trot: If it was a pantomime horse the front and back pairs would be exactly _out_of_ step i.e. the legs on the left side scissor inwards as the legs on the right side scissor outwards, then oppositely. The diagonally opposite pairs that are _out_ do the work, hitting the ground and swinging over the foot then lifting off the ground in their furthest back position.
- Canter: The left front and rear go forward in step as the right front and rear go back in step, so there's a definite side to side sway.
- Gallop: Both rear legs scissor forward together as both front legs scissor back together, the forward legs hit the ground and swing over the foot raising off the ground at the rearmost position and swinging forward for the next stride.
- Four Legged Hop [Pronking]: Speaks for itself: hop vertically in the air on all four legs at once.
Thanks also go to Junghak Na for the excellent horse applet-animation that I stared at in awe for hours; and the Illinois State Museum - Brookfield Zoo Museums in the Classroom for housing Eadweard Muybridge's classic photographs, The Horse in Motion from 1878. (edit August 2008: The Horse in Motion can be seen on Wikipedia: Eadweard_Muybridge)