I hit some problems with the new game mechanic, it bogged me down for several days.
But since then I made a lot of progress, and the new feature is not far from being ready. As promised, here’s a video that shows exactly what it is: http://youtu.be/oYQKEn_y1xs
It’s a long one, because I couldn’t resist teaching that naughty bear a lesson :-)
In 1.21 I made massive changes to physics and creatures locomotion, all to support the new mechanic. These changes mean it will take some time until I get them all tested and all glitches smoothed out. For example today I discovered that due to the changes pressure plates no longer work :-)
This video does not tell the whole story. There’s a few new, unannounced animals still to come in 1.21. Hope you’ll like them.