Sponge is a Forge core-mod that provides a cross-version API for modders.
The general idea is if you build your mod off the Sponge API, which will rarely change, then your mod will not need any updates when Forge/Minecraft updates. They are still in early beta but are making good progress. They work closely with Forge to make sure everything goes smoothly.
Last I knew Sponge did not have the parts of the API for adding things like mobs ready yet, but it is in the roadmap. Let me know if you want me to submit the bug report over there.
Thanks for the quick response!