NCF – Extended list of Neutral creatures.

It’s best not to change anything in the data.pak file itself. Create a mod instead. To do this, extract the edited file and pack it into a zip file, so that the structure of your zip archive mirrors the structure of data.pak. That is, to access the file, you would need to open the same folders, but inside a different archive.
Change the archive extension to h5u and place it in UserMODs.