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)