Because Doom was made before the use of 3d models in FPS games, every action a character did had to have 8 different sprites for each frame of action (That's a lot of sprites to draw by hand) depending on which direction the character was facing. I had the brilliant (or delusional) thought that the character didn't have to be doing the same thing in each direction. So if the Shotgun guy was facing the player he would do a wobbly advance, if he was running away he would be doing a Tasmanian Devil tornado, and if he was going to the player's left he would be doing the above pictured gallop.