It is indeed more common to leave out the "despre", but it's not wrong to use it.
By the way, here are some small corrections:
- Eu le-am visat pe animale. = I dreamed about the animals.
(here you refer to specific animals; maybe they were previously introduced by the context)
- Eu am visat (despre) animale. = I dreamed about animals.
(here you refer to animals in general)