Other than the eight Greeeds shown in the show, there are several other 'Greeed worthy' characters within the OOO storyline, even non cannon parts. Let's start talking about them, shall we?
Number 1, Nobunaga. This is confusing as several theories have popped out here and there. Is it a Greeed thinking it's Nobunaga? Is it Nobunaga revived? But we can skip that. Anyways, he is the Armored Warrior Inhumanoid who uses the Black Core Medals and appears EXCLUSIVE to Movie War CORE. As implied, Nobunaga is as smart and strong as his original, buying over the company he worked in within the movie itself. Wanting to kill the descendant of someone who killed him and that person who he wants to kill is the one he fell in love with, Yoshino Akechi, the ballerina. However, he also gets the Birth Driver which gets passed to Gotou by the end of the movie. However, the Black Core Medals are thrown into the mix by Dr. Maki which leaves Eiji with no choice other than to kill Nobunaga, sad in his friend's death as he dissolves and the Black Core Medals start to fly off to form Kamen Rider Core. Anyway, Kamen Rider Core is not really a Greeed not because of his Kamen Rider title, but the fact that he uses a Gaia Memory and the Black Cores to form himself/itself.
Secondly, the Shocker Greeed, rather obvious it is, made from the reverse engineered Cell Medal. Now there's this theory why the Shocker Legs is a repaint of the Condor Legs because the Cell Medal which was used was the Condor Cell Medal and the Shocker Eagle theme, but they used a Condor, close enough. He appears in the OOO, Den-O, All Riders: Let's Go Kamen Rider Movie, and he is a tough cookie to beat! Unlike previous Rider Kaijins, he manages to defeat the original Double Riders, kicking the *bleep* out of them and manages to successfully brainwash them. Thank goodness everything is fixed. Moving on!
Now, third! Gara! One of the four alchemists who created the Core Medals, the previous OOO. AND LET ME GET THIS STRAIGHT. GARA IS A MALE! ONLY THE HOST IS A FEMALE, GARA IS A MALE!!! Gara's Greeed form is considered the monster form and that, humongous, thing that he transform into. Appearing in the WONDERFUL Shogun and 21 Core Medals, Gara pretty much is a badass like all other movie Riders, Eternal, Ryuga and the sorts. Note that Gara uses no Core Medals in him... I think. *Drumroll* LAST. ONE.
Last but not the least! (Way overused saying.) We have Poseidon! Yes, the lovable badass. Don't blame him, he's good, it's just the Core Medals. Now, I don't like the idea of him being a Greeed, so the best idea is to introduce a SIC Poseidon figure and go all, oh he's the full Greeed form. So yeah, you've got that idea down. Appearing in the Movie War Core, he has the Same (Shark), Kujira (Whale) and Ookamiuo (Wolf Fish), what can go wrong with a badass combo like this?
Anyway, ending this here, I'll go off.