I don't know about a hack, but you can do it yourself.
First use Winrar or Winzip to get to the following location:
x100_000.z2f\entities\units\animals\ai or
entities\entities\units\animals\ai (the first is if you have ES, the second if you don't). You'll find a bunch of .xml files. There are seven, I believe, per animal, but you want the one with just the animal's name (AntelopeSableGiant.xml for example).
Open the XML file in Wordpad/Notepad.
Find the assortment of lines -- it's not very far down the page -- that says (this is the set for the Sable Giant Antelope):
It doesn't like Grassland very much at all. So change the "-5" to 5 or 10 and it'll like Grassland just fine. Save the file and put it back where you found it. That should work for you.
![:happy:](https://zooadmin.org/Archive/Smileys/default/happy.gif)