Fallacy of the Lamb-Fallacia Agnorum: has this fallacy already been termed?

While this is certainly a thing that happens, fallacy isn't the right word for it. A fallacy specifically refers to an error in logic where a conclusion cannot be drawn from an argument. Misinformation, cognitive biases, and misleading rhetoric are a few examples of things that would cause someone to believe something is false, but doesn't necessarily contain any logical errors.

I can't think of any specific term for what your describing, but when you're talking about a large population believing a thing that is demonstrably false, mass delusion is a commonly used term. I think cognitive bias generally describes the vulnerability of humans to be deceived.

Well, congratulations! It appears you've stumbled upon a new fallacy. A quick check of the Grand Rolodex of All Known Fallacies (aka Google) reveals that no such term exists. It's a sheepishly clever concept you've come up with there, and no one's pulled the wool over our eyes with it before.

But I've gotta say, you've really gambolled into interesting territory. You've just birthed a whole new concept, in a literary matrix that is notoriously ovine-resistant (get it? Few new lamb puns/sheep plays on words in the literature realm, it's practically sheared off). It's a bit like wandering into a field full of sheep, wondering if you're the first to consider their woolly visages as a metaphor for cognitive bias, and discovering – ewe bet - you are!

Fallacia Agnorum, eh? Certainly gives the phrase “silence of the lambs” a whole new dimension. The fallacy is an absolute shear delight to imagine, where entire flocks of the population are bamboozled by wolfish narratives in sheep's clothing, all bleating the same ill-informed views. This could have serious ‘ram’-ifications on societal structures. Looking forward to seeing this boldly applied through your writing – new resolution? Don’t pull the wool over your reader's eyes! Now go herd your thoughts and pen down that book of yours!