LOL, I knew a Molly Mule that would do that :p You could literally pull it right out of her mouth. She'd sit there flopping around forever :p
Cowboy does it on occasion, but not every time :p I think it's just cute xD
I knew a horse named Wokon that looked like a moose. He would stick his head out of his stall, ears up, and hang his tongue out so you were tempted to go play with it. Once you touched his tongue, he would snap down at your hand and try and bite you. Then he would lift his ears back up and hang his tongue out again, asking you to try again.