So far most of Usopp's lies he told while on his home island have come 'true' to a degree and in some fashion.
ch 24
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This one was the biggie. it acutally covers a lot. First off =
1- the Giant Goldfish.
We actually see two types of giant goldfish. There is the recent fighting fish at Dressrosa. But first one to show up was many, many times bigger - the Island Eater (as big or bigger than many sea kings)
[hide] ch 129 [/hide] it fits the next part of the tale
2-It's feces so long/big it was mistaken for an island.
Dorry & Broggy state the same about the Island Eater
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3- The defeated giant goldfish is used to feed a country of dwarves.
Here its is not Usopp that defeats the fish but the dwarves themselves. But yes, they do eat the giant goldfish native to Dressrosa.
There are two things here that haven't came true yet=
In that tale, Usopp traveled at age 5 and fought a southern giant goldfish. As we seen the tales do not come true to the exact match of the tale but in a mismatch and at different times.
This means =
1- There is a possiblity that we could learn that Usopp did sail of on adventures when he was young. (maybe that's why he's so scared, went out to sea too soon and saw how dangerous it was and how unprepared he was.)
2- a Possiblity he fights someone/something from the South Blue.
ch 40
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Let's see, a cerebus was met during Thriller Bark, the dragon (plus a 2nd one) in Punk Hazard.
I wanna see a 'alien store owner'!
Interesting thing is that Usopp may not remember the things he's predicted in his tall tales. ch 129 showed "giant goldfish" sounded familiar to him but he couldn't remember where he 'heard' (said it himself is more like it) it from.
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I believe there was one added in anime as I can't find it in the manga. I seem to remember the tale in ch 24 happening different in the anime. Instead of being interrupted as he told how he sent the fish to the dwarves, I think in the anime he was about to start another tale as I remember him saying "Did I tell you about the time I fought a giant mole?" when the Luffy & co interrupted him.
Of course this wasn't actually Oda's foreshadowing the fight between Usopp and ms Merry Christmas as the manga was already covered it and it was done in anime. But I added it here because I've forgotten that in the past and added to the list of "Usopp's tales that came true"