Did you ever see an unhappy horse? Did you ever see a bird that has the blues? One reason why birds and horses are not unhappy is because they are not trying to impress other birds and horses.
- -- Dale Carnegie
To answer his first question, actually, yes, I have, but they tend to resolve their issues quickly by dumping them, running from them, biting them if the flick of an ear or tail does not warn them off, or pasting them one with a handy hoof, and then they are not unhappy any more.