One specific type of mushan was only very briefly flagged as a tameable crocolisk, but nonetheless won over numerous hunters' hearts.
A couple of yaks were briefly tameable as part of the goat family. This goodbye is the hardest for me personally, as Cadmus the red yak will thus far be unable to migrate into a mount form.
Manglemaw the baby crocolisk and a group of tiger cubs were also briefly flagged as tameable. Gigantic baby pets have thus far been unintended, so it was unsurprising that they were not an exception to the rule. On the plus side, both shall live on in minipet forms. (Chuck is the adorable equivalent of Manglemaw.)
This is the unfortunate downside of beta testing and getting attached. But better to have loved and lost than never to have loved at all. Farewell, you kings and queens of beta. You shall not be forgotten.