"Critters of Moria"

[Boromir88]

That's an interesting quote. Sauron was created at the beginning, and these creatures are older than him? So there are some creatures/beings potentially older than the Ainur? How do they fit with Eru then?

Well, either Gandalf's wrong. Which is quite possible, as we are aware of what Tolkien feels about his characters, and what his mind is. He uses Treebeard as an example, and says that Treebeard's a character in his story, that's not his own thoughts. So, Gandalf here is speculating, and his speculation is wrong.

Or, perhaps he means they're older than Sauron, as in his time on Middle-earth. They were there in Moria long before Sauron first set foot on Middle-earth?

Or, it's what tgwbs has said.