There's some sort of bug where some fresh tames will be flagged crim or aggro or something like that, where the new pet's status as your tamed pet doesn't go through all the way (something like that). The health bar and name will be highlighted grey on the fresh tame when this happens. It's a fairly recent problem, it started within the past month or so.
It happens with both honor tames (where the pet has never actually tried to attack you) and otherwise.
My usual way of dealing with it if i've tamed something in a guard zone is to mount it and get it the heck out of town and keep it out of town until the color of it's health bar updates correctly. I have also seen it work (on a guilded char anyway) to say "all guard me" and make it take a step. The bar should turn green-colored when you do that. I have never tried that particular one in a guard zone, though, so I can't say I entirely trust it there.
There's other weird bugs with pet flagging right now, new ones. And you can even get yourself guardwhacked by releasing certain pets in town and calling guards right away. I released a bake kitsune in town and called guards, and then I got guardwhacked, and then the bake (which was still guarding me) attacked the guard, and they had a brief fight (the bake lost). I was still linked to the pet even though I had released it, and I could see the damage amounts that the guard did to the bake as the guard killed it.
