This is the other side to the previous chapter.
When Sento and Banjo combined while using the Crosszbuild can, the property's of the genius bottle and the rabbit fullbottle(and by extension the tank fullbottle) made a manifestation of Sento's genetic makeup and power, which was transported into the world of Monster High, into the womb of Frankie Stein(dont ask how that works). This causes Frankie to have dreams of Sento, as well as Banjou by default, when he transforms into his rider forms, as well as the final scene of the show with the rabbit dragon form done by Sento, Crosszbuild against Blood(aka love and peace finish), and Rabbittank sparkling, rabbittank hazard, Rabbitrabbit and Tanktank.
Frankie soon starts to feel sick and begins getting fevers, leading her to have a check up by the school nurse, who cant see what's wrong as shes never worked on a Cavadier, so Ghoulia checks, who reveals some form of powerful energy was developing in Frankie's womb, and was likely going to emerge soon, this of course raises many red flags with many people as to why and how this happened, plus how it was possible, as well as people making rumors and making assumptions. The next day Raven suddenly begins to get covered in red and blue sparks as she becomes stiff, and the spot where her womb is begins to glow bright red and dark blue. Shes taken to the nurses office with Ghoulia, but when she gets there the sparks and glow suddenly stop, but then the red and blue glow returns violently as it begins flowing out of her stomach and taken what can only be assumed as a humanoid form, as it completely forms a voice is heard from it, 'RABBIT, TANK, BESTMATCH!, ARE YOU READY, FULLMETAL MOONSAULT, RABBITTANK, YEAH', the figure then gains a more solid and physical form. Frankie recognizes the figure from her dreams, Kamen Rider Build, however the figure just looks around confused, unaware of what's happening, who everyone is, or even who or what he is, having the mentality of a child as if was technically just born. However Draculaura took the liberty of pulling out the Rabbit and Tank fullbottles from the build driver, unforming the armor, leaving only a small child with Frankie's hair colors and  glowing red and blue eyes.

There are very few fanforms for build online involving just the rabbit bottle or tank bottle, so I guess it's good how many forms for those two he has in the series

Ignore the Rogue form