In the opening of season 2, there are dinosaurs and prehistoric animals there. Why?
I strongly suggest Rule of Cool. There are no dinosaurs or other prehistoric animals in the story, neither anime nor manga. But dinosaurs are damn cool.
If I have to try to find a connection to the series, the theme would be extinction. If all walls would be breached, then humanity would face extinction through titans, right?
Manga spoiler (don't read for more enjoyment of the series): With further manga knowledge, I would reformulate the above answer to
Humanity inside the walls could face extinction in the near future.
It is because the beast titan look like a gorilla or ape. Also the beast titan could be the mix of a prehistoric animal with a titan and he can probably control animals just like
Eren can control titans or call them.