Pinky the Chihuahua is Isabella's pet dog, and, like Perry the Platypus, is a secret agent for OWCA.
Pinky is shown to shake all the time - his nemesis even comments on it in "Isabella and the Temple of Sap".
Do we know why he shakes all the time?
Chihuahuas are notorious for shaking and shiver more than other dogs. So much in fact that inexperienced owners oftentimes believe a more serious health issue is causing this behavior.
One of the most common reasons why Chihuahuas shake and shiver is simply because they are cold. With an average weight of just 2-4 pounds, Chihuahuas are one of the smallest dog breeds and the world; therefore, they are more susceptible to the effects of cold weather. Just like the we instinctively shiver in cold temperatures, so do Chihuahuas. It's their way of speeding up the flow of blood throughout their body so they don't fall victim to hypothermia.
Also...other reason are given (in the link) but the primary reason is...
The obvious reason why some Chihuahuas shake is because they are cold and their bodies are reacting to the ambient temperature much the way a human body does. Unless they are the long-haired type, Chihuahuas have very short fur and you will frequently see them sporting sweaters and jackets since their owners want to keep them warm. The Chihuahua has an unusually high metabolism, and their tiny bodies lose heat rather quickly.
Because Chihuahua do that in real life too and mostly there is no serious concern there. From petchidog.com:
It is not uncommon for a Chihuahua puppy or dog to shake. Some refer to this as trembling or shivering. In fact, so many Chi's shake that some mistakenly believe that it is a trait or characteristic of the breed. It is very common, however there is always a reason why, and there are steps that you can take to stop or at least decrease the shaking. -
Ruling out Health Conditions which cause it :
If a Chihuahua has sudden and violent shaking, one must rule out poisoning. This may include the ingestion of certain foods (chocolate and raisins are two toxic foods that dogs may ingest), or other common poisons including household cleaning solutions, insecticides, or human medication.
While this must be mentioned as a possible reason for trembling, kidney disease or kidney failure is a very serious health crisis that involves many more signs aside from shaking. Some of these are: pale gums, mouth ulcers, bloodshot eyes, an odd ammonia smell (due to a buildup of waste products in the body), weight loss, a dulled coat, weakness, difficulty breathing, disorientation, and loss of balance.
If you suspect an ear infection, veterinary care is recommended. A Chihuahua's ears may need to be flushed out and medicated drops given.
When a dog is moderately to severely injured he may shake uncontrollably due to the shock of trauma and/or pain.
The Most Common Reasons Why a Chihuahua Shakes
Low Blood Sugar
What happens: This breed tends to have a high metabolism and in turn is prone to dips in blood sugar levels. A minor dip can cause shaking. A severe dip will cause other symptoms including drowsiness, confusion, and even fainting.
To help prevent this: Chihuahuas of all ages should be fed small, frequent meals and snacks. Most Chi do best with 3 small meals per day and 3 to 4 snacks. Distributing the food in this way allows the body to receive a steady supply of fuel and nutrients.
What happens: This breed - due to its very small size and low fat-to-muscle ratio - has a hard time regulating body temperature. Even if you feel just fine, there is a good chance that if your Chihuahua is shaking, that your dog is feeling chilled.
To help prevent and remedy this: Keep rooms warm, try to keep your Chihuahua out of drafty areas, and put clothing on your puppy or dog to see if this stops the shaking. Even just a light shirt or sweater may be enough to insulate the body and prevent shivering. n the winter, a lined vest or coat can be worn outside. When it is raining, you may find that your Chihuahua does better when protected, since a wet coat can cause a deep, penetrating cold.
Even twitching/shivering can be a symptom of seizures in Chihuahua. Refer this link for details.
So for Pinky it seems to be a stereotypical representation of the Chihuahua breed.