I believe you are a
Cat. More specifically, a housecat
I noticed that cat and kitty appears twice in the riddle, and it is tagged history--plus--your profile picture is a cat. So I Googled "evolution of cats", and this was the first hit:
- from here
I think each line in the riddle is referring to one of the eight lineages.
When I was one, I hid my species. This is referring to the Panthera, and "I hid my species", I believe, is talking about the rareness of their fossils, as you can infer from this excerpt from here: (I've shortened it) "The pantherlike ancestor to the cat family...was difficult to trace..., because feline fossils are rare, and difficult to tell apart." It's from the same source as the image, so I'm confident that's what you're talking about. A Panthera:
When I was two, I was still light. I think that light in this context implies weight. The second of the eight lineages is the Bay Cat, which, according to the image, is a small to medium size cat, between 2 and 16 kilos in weight.
When I was three, I gained weight. The Caracal, the third of the eight, weighs between 5 and 25 kilos, thus implying the gained weight.
When I was four, I was many. The Ocelot, the fourth, lived in a broad range of habitats, while the previous ones were restricted to different continents.
When I was five, I was a kitty. This is talking about the fifth, which is the Lynx. I suppose that "I was a kitty" is referring to the housecat-like face of the Lynx, apparent here:
When I was six, I was a bigger cat. The Puma (sixth) is a diverse cat, according to the image, with some (cheetahs) being able to grow up to 65 kilos, larger than the Lynx's max weight of 20 kilos.
I skipped seven, no time for that. Less work for me then :). But the seventh lineage is the Asian Leopard Cat:
When I was eight, I became a plague. Okay...this, I believe, is referring to housecats, which are the eighth lineage, and according to Wikipedia, it's estimated that there are between 200 and 600 million housecats in the world, hence the "plague" of cats.
I never reached nine, I'd say that's fake. Since the eighth lineage (housecats) is part of the current time period, and the ninth doesn't exist yet, "I'd say that's fake" is referring to non-existent cats, which are, of course, fake.
Also, @FlanMan said in the comments that the last line could be related to a cat not having nine lives, so maybe it's that cats haven't reached their ninth lineage yet...?