Question about the for the horde achievement

I also have all of the fossils, forgot about that one

Have you looked where you can see what all you've gathered? It's a jewel icon, (I think, I'm not in the game), that's located in your Sims inventory. From there, you can see every collection and what you've gathered. If something is grayed out, that means you still need to collect it.
Edited to add that you can also see all of the collections, so from there you will also see which one you are missing. :)

I think it might include the sugar skull and easter egg collections too. And they're not unlocked at the moment, because they were only available for a short time in-game. I think you could probably find them in buydebug though.

I'm in game because I just had to go look, lol. Here are all of the collections, there are 22!
Frogs, gardening, mysims trophies, metals, crystals, elements, postcards, fossils, microscope prints, space prints, aliens, space rocks, fish, insects, geodes, decorative eggs, experimental food photos, voidcritters, sugar skulls, city posters, snow globes, and holiday cracker plushies. I think the one you're missing is aliens. They're the little "animals" you can find when you travel in the spaceship.

I just discovered that the city living collections are included in For The Hoard. I don't know if this glitch occurs if CL is not owned, I have yet to look into that.

Is there a list of all this stuff somewhere?