Copy or create child when editing dinos?

I have noticed that certain dinos have odd behavior when they are copied into the mod folder rather than creating a child.

The iguanodon aren’t be able to move after you switch between 2 leg and 4 leg mode if a copy is made. If a child is made then it works fine.
Terror birds have drastically lower sprint speed when copied as well.

The tutorial videos I have seen on creating custom dinos have all used the copy method. Which is correct? What are the major differences? Thanks in advance.