Nupro makes a yummy liver gravy that many dogs love! Nupro Silver is the kind with joint support, though it may not be enough to increase comfort in an already painful dog. Springtime has various levels of joint supplements, some of which they say are good for dogs with more advanced arthritis. I can't really vouch for anything's effectiveness, though, but I did have my last dog on Nupro Silver starting when he was a year or two old, and he never had any issues in his limbs. He did have a neck issue at the end, but I think it was a disc thing. Hope you find something that helps!
First time Chihuahua owner with many years of dog experience!