In a well known fantasy series, giants appear half-way through the third book. And it turns out that all the characters in the books knew about giants, while the reader had never heard of them (obviously because the author invented them only half-way through book three). Don’t do this. This kind of clumsy world-building can shatter the illusion that the world the characters are in has any solidity of its own. If you want to add something later, figure out a way to weave it in earlier, or don’t add it.
This is something that I remind myself of when I'm developing a synopsis for a story. If there is going to be an encounter down the road with a radiocative gorilla then make sure you at least hint to the fact that there might be a radioactive gorilla. Yes, I'm still writing about radioactive gorillas.
TonyDoug Wright is the author of two webcomics titled The Red Devil and Day 165. Do him a kindness and check them out.