Forcesave is an old mod, now included in most, that lets you save creatures or other editor created content without an error. Other creations, like adventures, cities, tribes, and other planner based items with mods do not require force save.
Spore is creative, even the mods and hacks.
Or, should I say, especially the mods and hacks?
Now currently making content for the Life is Strange and Portal communities. Check me out on YouTube