Seeing as the Beast Pirates were susceptible to spies joining their ranks or affiliates (X Drake and Kyoshiro), other factions may have sent undercover agents to join the Beast Pirates. One person I can see stationing a member of their crew within the Beast Pirates to keep tabs on Kaido and his actions is Doflamingo. It has been established already how shrewd a man Doflamingo is and the extents he goes to in order to protect himself and his status. Information was one of Doflamingo’s most powerful weapons. It allowed him to become a Shichibukai and to keep the World Government at bay and semi-subservient to him (using the CP-0 to trick Law into believing he quit being a Shichibukai). Doflamingo had Monet go undercover in the Dressrosa Palace prior to their invasion and on Punk Hazard to keep eyes on Caesar, a partner in his dealings with Kaido. Given all that Doflamingo has done, it seems natural he would also have had someone positioned at one point in time undercover in the Beast Pirates.
The prospect of Doflamingo using an undercover agent seems more logical when you consider Wano Kuni is a closed border nation where no information involving the actions of the Beast Pirates would reach Doflamingo directly – Kaido residing in Wano Kuni was not common knowledge illustrated by the surprise reactions from Law and the Straw Hat Pirates when Kin’emon revealed Kaido and his Beast Pirates were in Wano. Only the information Kaido (or King) would want Doflamingo to know would be conveyed to him. If Doflamingo desired unfiltered information regarding Kaido’s movements, he would need to acquire it directly from within the Beast Pirates. Additionally, I believe rather than succumbing to the rumours of Kaido, the fear Doflamingo has of Kaido is sourced from direct knowledge of Kaido’s actions and the forces under Kaido’s command generated by the information he received from the agent undercover as a Beast Pirate subordinate. As for who Doflamingo would have sent, they is only one person with the skill set able to perform that task – Monet. Alongside her role as Caesar’s secretary, Doflamingo may have also had Monet acting undercover as a Beast Pirate. Such an act would explain the Harpy form Monet adjusted to.
If Monet, in her human form, worked her way into the Waiters rank of the Beast Pirates, she would have been given the opportunity to consume a SMILE to become a Gifter. But rather than consume a SMILE and risk the side effects of it (and potential death at consuming an artificial Devil Fruit on top of the Yuki Yuki no Mi), Monet remained a member of the Waiters until Law fortuitously appeared on Punk Hazard. Before the SMILE production began, Monet may have begun her mission undercover within the Beast Pirates where she was able to provide Doflamingo with information concerning the general movements of the Beast Pirates. At present the Beast Pirates number 20,000 pirates (not including the 10,000 members from Orochi’s forces) and even in the past, twenty years ago, when Oden challenged Kaido, the Beast Pirates were still a massive pirate organization with a thousand members. If Monet had infiltrated the Beast Pirate ranks sometime in the past and she was one among thousands of lower ranking members, it would not be suspicious if she wasn’t present on Wano Kuni or Onigashima Island all the time considering keeping track of all those members isn’t a process the Beast Pirates have shown to implement. The fact that 5,400 Samurai forces were able to infiltrate Onigashima Island with just a change of clothes illustrates the absence of an identification system by the Beast Pirates. As long as an individual looks the part, they are recognised as part of the Beast Pirates. In view of that, a lower ranking member being present only sometimes wouldn’t attract attention and suspicion. This could have been how Monet was able to carry out her role as an undercover agent within the Beast Pirates without having been present in the crew all the time.
Once the SMILE production commenced, Monet’s focus shifted toward Punk Hazard as she could not work her way deeper into the Beast Pirate organisation without possessing a Zoan Devil Fruit or once it was introduced, a SMILE Fruit. Monet would not have told the Beast Pirates she already possessed the Yuki Yuki no Mi similar to how she kept it a secret while working in the Dressrosa Palace. When Law appeared, Monet found a way to rise above her standing as a Waiter within the Beast Pirates without attracting needless attention to herself. Using Law’s abilities with his Ope Ope no Mi, Monet potentially requested Law to change her arms and legs with that of a bird in order to give a certain faction the illusion that she had consumed a SMILE thus allowing her to rise to the next rank within the Beast Pirates – to the level of Gifter. Such a position would have enabled Monet access to additional information not available to the members of the Waiters or Pleasures.
Without proper reason, Monet’s decision to remove her functioning human limbs in favour of wings, talons and a bird tail seem incomprehensibly odd. I find it hard to rationalise the reason being merely because she liked birds and subsequently desired to fly. It is just suspicious no reason was given. Nothing at all. Monet is a very methodological individual whom takes her duties seriously. If she were tasked with an undercover role within the Beast Pirates, she would without fail have realised the opportunity Law possessed once he appeared on Punk Hazard being a doctor who possessed the Ope Ope no Mi. This could be why upon realising what Law could do, she suggested Law help heal the injured former Punk Hazard prisoners. Monet wished to gauge Law’s capabilities before allowing him to conduct surgery on herself. Ultimately, Monet using the Harpy form to work her way deeper into the Beast Pirates sounds like an undertaking consistent with Monet’s character and role within the Donquixote Family. If Monet really was an undercover agent within the Beast Pirates, could she possibly make an appearance during the Wano Kuni Arc? I wonder.