First, Flash ambled into Winston's stall. He came back out after checking to make sure the gate was in working order. He didn't seem too impressed or interested in Winston's dirty stall. Flash leaves his manure in one or two neat piles. Winston scatters his everywhere,
Meanwhile, Winston gleefully went into Flash's stall. And stayed there.
...until Flash went in and chased him out.
Once Flash left the immediate area, Winston was back for more. He's a typical five year old, wanting to play the game over and over and over again.
Flash, being the grumpy old man alpha horse, doesn't have to do much more than wave his head at Winston to get him to skedaddle outta there.