They are robots, but...
They can get physically sick from viruses. There's a substance called "Bioter" inside the machinery that fuels the movements of the creatures. The bioter is oil-like outside the cahracter but more plasma-like inside. Mechapets heal quickly, but traces of Bioter can be seen leaking momentarily after a wound or cut is created. Bioter can handle changes in pressure and pH but may struggle with temperature shifts. Especially cold temperatures might cause stiffness and even acing in narrow parts of the body, like the fingers. Mechpets have internal parts that are coated or made out of their bioter, making the parts more sensitive and very organic-looking. This is how Mechpets can taste foods with their tongues. These parts are never unintentionally visible.
A form is humanoid, but...
They are rarely msitaken for a human. Especially up close. Mechpets can use and wear all sorts of accessories. Some even choose to go without, but it's common for Mechpets to have some sort of outfit in their A-form. Othervice they would be considered naked.
MechPets don't neccessarily need pouches or bags since there's usually a hollow space under the plating that they can store items in. Some larger items, like weapons, they do need to carry.
Whatever the physical look of a Mechpet, they always have some visual way of depicting their emotions. Be it a pixel display, eyebrows, lips, or expressive ears, Mechpets are social creatures. Most Mechpets can talk, but some don't or choose not to, so being able to display emotions in other ways is required.
All Mechpets have a brain module that has limited space for memory and sores their personality and knowledge. The brain module can be wiped clean, but othervice Mechpets can't really die .Intellectually, they are on a level with the humans, and most humans know that.
They can transform into animals, but...
They have fur-covered plating that hides seams well, but they are fully robotic. so some Mechpets resemble more fuzzy animals in their F-Form, while others don't. The character can be any color and can be based on any type of animal, real or fictional. Depending on the animal, The mechpet may be able to blend in seamlessly, especially from humans.
Extra limbs, extra eyes, visible mecha parts, and gadgets are all present in mechpets! The character does not need to have a tail or ears. The only required mechanical part is the Wigs.