I created a DK just to have it sit near the rare mob in Silverpine Forest that dropped the Black Tabby (before account-wide or Cata). I would log on and see if it's there. I got lucky and it dropped. In the process of mailing it, I right clicked while the mail window was on the inbox, accidentally learning the cat. I was pissed. Over a year later, during Cata, I was cleaning my characters out and thought nothing of deleting that DK.
Then account-wide pets came. I remembered that stupid cat.