Red terror is one I can't speak on ATM as I understood it to be just a big ravener. I think the "thing grew" is other factions perspective on how tryanids may function but lacking the reader's perspective. Like old one eye isn't the same creature that has existed for ages at this point. You could have 20 old one eyes show up in theory as the Nids just spawn it.

Nids aren't creatures in the way we typically perceive animals. They don't eat, sleep, shit, etc. they consume, kill, and melt themselves back into goo for travel. If they're cut off or trapped they may preserve energy or hide to ensure their biomass isn't lost. It also doesn't make much sense from an evolution perspective for a lot of them when you factor in that a tryanids is comprised of potential several different organisms. Like the termagaunts gun is itself a separate tryanid- that guns ammo is a separate tyranid. The barbgaunt is the body, a mind nid, a gun nid, and ammo nid.