Onyx Dragon by Ruby Ryan

Orlando didn’t mean to rope Cassie into this. At least, not without warning her first.
If only she had listened.
Cassie’s handling her new gryphon-related responsibilities about as well as a rejected organ transplant. How could she react differently when the man she slept with turned into an animal afterwards and flew away?
But the totem has made its choice, and fighting her feelings is a losing battle. Especially when she can’t get the memory of Orlando’s beautiful, dark body out of her head.

Following him onto a train bound for Texas seemed like her best option, until it’s hijacked by men with machine guns south of Chicago. But that’s not even the worst part: the hijackers are looking for her.
As they do their best to remain undiscovered, Orlando and Cassie are going to have to put their differences aside in order to stay alive. Because like it or not, they’re no longer in control of their destiny.

The Onyx totem is.