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Sounds like Tomato Surprise.
Right, but here it's not even a character but an object no one expects to be able to narrate.
...so it's Tomato Surprise still. The original example involved tomatoes
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Narrator All Along specifically says it's a subtrope of Tomato Surprise.
"Narrator All Along but for animals or inanimate objects" just strikes me as a needlessly specific sub-subtrope, unless you can find a lot more examples of it.
The All-Concealing "I" when first person is used to facilitate this sort of reveal by hiding information about the narrator.
The story's narrator turns out to be been something else all along, sometimes an animal but even an object (in a setting where animated objects aren't the norm.
There's a 40K comic where the narrator keeps talking about his accomplishments and who he served with over images of a Space Marine fighting, ending on how he doesn't want to end up abandoned as the Marine dies... and then it turns out it was his gun talking all along.
Edited by Chabal2