Given that your patronus (and to an extent your animagus form) are manifestations of your personality it would make sense that both forms could only be things you know exist.

Patronus can change so could feasibly turn into a dinosaur if you read a book on them or something if you learn about them, but I'd doubt someone who doesn't know what a dinosaur is would have a dino as a patronus. Espeically since a lot of what Victorians knew about dinos has turned out to be wrong
eg....

this is a victorian image of the elasmosaurus with the head on the wrong end....

What Victorians thoughts Megalosaurus looked like

What we now think it looked like


The same with foreign animals - if you've never heard of a kangaroo, goblin shark, or some other exotic animal your subconsious probably won't manifest as it. You'd need an image for your brain to copy.