My partner and I were returning to Las Vegas (where we are visiting family) after a weekend rock climbing trip. We followed I-40W from Flagstaff and had an uneventful (but gorgeous) drive through western Arizona. Sometime after sunset and near the confluence of I-40W and 93, my high beams caught a grey, flying creature flapping large wings and ascending quickly out of view. My first impression was that it could have been a large bird of prey, but I know of no native North American birds that would have as much surface area as this did. It followed an upward diagonal trajectory, and appeared to be featherless and tailed. I only saw it for 1-2 seconds. I second-guessed my eyes and asked my partner if she had seen something, to which she responded; "You saw that too??" I asked her what she had seen without telling her what I had; she also described a featherless, grey-skinned pterodactyl-like creature with a very long tail flying quickly upwards (though not straight upwards, on a horizontal upward trajectory). We had been joking about seeing a UFO since we saw the Uranus gas station and other UFO themed places on our way out, so it was surreal to see something neither of us could explain without sounding overhyped on the areas "vibrant" attractions. My partner and I were not under the influence of anything, though I had a cup of coffee an hour beforehand if that counts for anything.