As Elijah said creatures were temporarily disabled until creature combat is implemented to avoid cases where we have 500 rancors in a city, combat is switched on and they kill everything there before anyone logs in.
As for the creature inventory option, I would guess it was removed until they had worked out how to capture creatures, and that is implemented.
There is a common joke in the Combine that game features like this, and upcoming ones, will be here "soon." The joke is that sometimes the features are back in short order, and sometimes it takes a long time before they are returned or implemented. The game is supported by volunteer work only, so next time you see an Admin, or someone on the development team be sure to thank them. In the meanwhile, just be patient and enjoy the things we do have.