In shortA dog shakes for four main reasons: he is cold, he is scared or stressed, he is very excited, or he is in pain. In small breeds, trembling is almost a way of life. What settles it is the rest of the body and the context: a dog shaking while waiting for his ball has nothing to do with a dog shaking prostrate in a corner.
Emergency if the shaking comes with vomiting, drooling, pale or bluish gums, a drunken gait, seizures, or if he may have swallowed a toxin (chocolate, xylitol, rat poison, plants). Also consult for new trembling at rest, especially in an older dog.
Very small dogs tremble a lot: cold, excitement, fast metabolism. It is common, but still have blood sugar and temperature checked at the vet if it is permanent, and dress him in winter.
Often muscle weakness or arthritis, sometimes back pain. Age is not a disease: treatments exist that relieve. Have him examined.
Twitching, paddling and little cries during REM sleep are normal: he is dreaming. Do not wake him abruptly. Violent, stiff jerks with drooling and loss of urine, however, suggest a seizure.
Mimo reads behavior. It does not replace a vet.
Ten seconds is enough. On iPhone. Android is in closed testing.