Energy Cats
At their core an energy cat is a well loved vehicle, new or ancient. That love is what creates their souls, with some random detail of the vehicle determining what type of cat they are.
The energy cats begin with Muun, and eventually after a lifetime of travel, end up retired, taken care of by Won, until they fade away entirely. Muun is the 'first' energy cat, the origin of them all if you were. Literally their name means 'headwaters' in the original Thai. Won is their flipside, the final finish line for all the energy cats. His name means 'audience' not death, because he's not there to remove an energy cat but to give those that lose their physical form someone to be with so they don't get lonely. While Muun is trapped in this physical plane of existance, Won is forever bound to the plane all energy cats draw power from.
An energy cat happens in a moment of a vehicle brushes up against catastrophe, and what energy they have is vaguely based on something happening when the catastrophe occurs. Sometimes they avoid it, sometimes they get crashed headline into it. For example this is a moment frozen in time after something hapened during a race in a thunderstorm, but the car wasn't done yet, it was too well loved to give up... and instead of crashing to a stop there is a moment caught in time to bond with the cat... then resume BEING with that cat/car, lightning from the thunderstorm wrapped around the cat and car now.
When designing an energy cat think of what cat fits aspects of the vehicle (large or small, most are 'big' cats admittedly, Monsoon is 'abnormally small' since margays are TINY and the BRZ is considered a very small car these days) and the colors can be anything at all, with the "energy" (which is extremely vague on purpose) based on something happening when the transformation first occurs; it could be regarding the car directly or what drives that love for the car, etc.
There are exceptions, as with anything supernatural, it's less about a rigid template and more a journey to honor and remember a vehicle, a way for those of us that "just think they're neat" and may never have done anything exceptional TO our vehicle except treasuring that vehicle and all we've been through with it, to share that love.
As a footnote, "energy" doesn't just mean fire/lightning/etc, it could be plants, dust, oil, darkness, anything you can conceptualize as "emanating from something" somehow basically. Even exotic non-physical concepts like "time" could fit. Lots of possibilities and it's meant to let people explore a lot too and see how the energy cats fit their mind the best. It's not an innate power, it's something that 'leaks' from the cats as a reminder of where they came from, but that the cats can learn to control.