Spack

It's a portmanteau word for "spy cat." Because they are cats, who spy. That is their purpose. There are some differences between them and normal cats. Not big ones, because the important thing is that they still look and behave like cats. But they are not. They are spacks.
The primary difference comes from their observational capacities, and their fur patterns. They can, and do, meow in the normal way, and use body language in a manner which other normal cats seem to be able to figure out. But between themselves, and the person who knows how to read them, their fur can become intricate information about what they observed.
They are, mostly, still just cats. However, there are only some breeds that they can resemble. Has to be shorthair cats, because the fluffier ones didn't work well with the shifting fur patterns. They had to be at least two colors for the average person to be able to detect the differences in their fur. Their personalities are always meticulous groomers, more so than even the average cat.  They can only ever be a medium sized cat, neither one of the tiniest breeds nor the largest.
Their intelligence is a matter of some debate. They're able to pick up on a lot of information, going by what goes into their fur, but they lack the motivation to do anything with it. It's a bit like a parrot, copying what it hears without knowing what's being said, exactly. And for using them as spies, they would frequently observe things which were not at all useful.
As pets, however, they tend to be quieter, and have a great affinity for sitting in laps and less for knocking things over and scratching up the furniture.
Interestingly, the more the species progresses and comes into its own, the less they communicate in typical cat ways, moving more and more to exclusively use the fur patterns and thus a visual language.