Loque'hanak the Spirit Beast is not the rarest tamable beast in the game at the moment, but she is unique in her own way. She's the only member of the spirit beast family in play right now, tamable or otherwise. She's...well, you can see her looks, and she's got the family ability to cast Spirit Strikes. These hit for arcane damage right away, and then the same amount again 10 seconds later. (This is going to take some care, if I take her instancing, to avoid messing up crowd control.)
Special thanks to Sean Riley of Blogatelle for suggesting the name Ørlög, meaning "primal law", and the accompanying quote "To understand Ørlög is to understand the threads of wyrd." That's good for a spirit beast, I'm thinking.