Thirteen year old pappilion went from his normal extremely active happy self to a panting, non drinking or eating dog. He shows unusual habits as well (not laying in his favorite spots, not following me around like my shadow). I've tried placing water in his mouth but he just spits it out. This started about twelve hours ago and I'll be taking him to the vet if he doesn't come around by morning.

At 13, he might be ill enough to be preparing to die, I'm so sorry, and hope it's not so.
It's happened to me and countless others.
He may have tumors, especially if he can't even swallow water.
One quick and easy health indicator is pink gums, and they are white if sick.
My last old dog was given Prednisone for a few final days, and it helped him eat (hamburger, whatever he wanted the vet said), and even let him wander around outside checking his favorite spots. But he had to pee every 30 minutes, day and night, so I took him back to be put to sleep when I had to get some sleep.

13 is a good age for a small breed. As soon as they stop drinking, it's time for a vet intervention. In the mean time you can try adding a little chicken broth to his water, or offer him an ice cube, lots of dogs get a kick out of ice cubes. There isn't much we can guess on with out knowing your pups previous health issues, and as obvious as this sounds, we can really only suggest that you see a vet.

Will your dog eat wet dog food? If he will eat wet food, he will be getting a small amount of fluid into him, which will help avoid dehydration. Please keep us posted.