I believe Avalo Pizarro was a former Beasts Pirate member. Pizarro’s bio describes him as a tyrant king from the North Blue who was overthrown, giving him the epithet “Corrupt King”. He was then put in level 6 of Impel Down, “erasing him from history”, due to a number of horrible crimes.
The problem with this is that the World Government doesn’t seem to care about dealing with tyrants unless they end up being a threat to the Government itself. For example, Germa, another North Blue Tyrant nation, was considered a member nation of the World Government up until their alliance with Big Mom was revealed. This made the Vinsmokes traitors and therefore a threat.
Wapol, the oppressive king of the Drum Kingdom had to be dealt with by his own people + Straw Hats, and the Government didn’t care to help. He wasn’t a threat, and even got to rule another member nation of the World Government.
So there is more to Pizarro’s past if he needed to be erased from history. What if, after being deposed, he somehow met Kaido, a powerful pirate in the New World? Him committing crimes worse than tyranny obviously isn’t enough proof he was a Beasts Pirate member however. This theory mostly hinges on this image we got of Kaido and his All-Stars when Kin’emon was telling the Straw Hats the story of Oden.
Notice that this image was set 20 years in the past and we are shown the silhouettes of who we assume to be King, Queen, and Jack. Here’s the problem: Jack is currently 28 years old. During Oden’s battle against the Beasts Pirates Jack would have been 8 years old. This means the figure we saw resembling Jack was not him.
I believe this figure was Pizarro, whose wide-set horns and long hair fit the silhouette in the flashback panel. His age fits as well since he’s currently 42, putting him at 22 during Oden’s fight against Kaido. His age implies he was a relatively new member of the crew, since his time as a king likely would have occurred prior to joining Kaido.
His epithet “Corrupt King” could be a hint to his position on the crew, the former “King”. Perhaps he was captured sometime later and the Beasts Pirates either failed to rescue him or didn’t bother, leading to his imprisonment in level 6 as a high ranking member of an Emperor’s crew. The All-Stars we know as King and Queen were promoted to their current positions and the man we know as Jack took up the newly vacant All-Star position.
*Theory by UltraBananaBuddy