There is more than one type of Overlord Invess. The one Kaito encountered is the Western styled one. Here's the Chinese theme one.
DJ Sagara gives Kouta the Kachidoki Lock Seed, but Ryoma has not even heard of such a Lock Seed before.
Okay, so first off, that invess looks fuuuuuuukkin dope. I love all the designs for this show. They seems to distill some of the best designs from all over the history of rider into a few really iconic, original looking, designs.
Now as far as what this means for the story, I'm beggining to wonder what their background is, and I see 4 main possibilities.
1. They are the gods or creators of the invess/the forest. I see this as the least likely because they don't have quite the aura of that kind of big bad, and with their designs being so linked with the rider designs so far it sees like their role would be more similar to the riders (this new one is a dead ringer for ryugen).
2. They are the first invess. This one seems possible, but at the same time I don't see a story in it. Doesn't seem like urobochi's style.
3. They are the same as the riders, but for a different civilization, on a different planet. The forest affected them differently because they either didn't develop any technology, only directly consuming the fruit, or they developed different technology from the lockseeds and drivers.
4. This is the theory I personally like the most, and it seems the most likely to me. The overlord invess are the eventual form of the riders. If you use the power of the lockseed too much, you eventually become a powerful sentient invess, these powerful sentient invess being the overlords of the newly created forest. The riders are the monsters.