1500 years ago, Princess Celestia fell in love with a pony named: “Yaman Miek”. They loved each other beyond everything in the universe and the population of Equestria couldn’t be happier that their princess had found her special somepony. However, little did they know that Yaman was actually an ancient being named: “Yamantano Mikaboshi”. Whose only intention with marrying Princess Celestia was to inpregnate her and then create an army of children that could take over the planet. Those children would be able to manipulate their genes, which them would allow them to inpregnate each other and then create another generation of healthy babies.

However, thanks to Clover the Clever and Starswirl the Bearded using their magic on Yaman on his and Celestia’s wedding aniversary. Around four months after Celestia was impregnated, his secret was finally discovered. Yaman transformed into his normal self (see picture). He became just as scared that he was exposed, as everypony was upon looking at him. Since they had never seen anything like him in their entire life (including Princess Celestia). Yamantano desperately tried to explain that he was under a curse, but when Clover used another spell to make all of his thoughts become hearable. So that everypony learned that he only used Celestia as a pawn. This broke completley Yamantano and he decided to force Celestia back into a pocket dimension. But Celestia managed to break out and fought him alongside her sister, Starswirl, Clover and the Pillars.

After a violent and fierce battle, Yamantano was seemingly killed by the combined force of Celestia, Luna, Starswirl, Clover and the Pillars. But in reality, he had just managed to escape back into his own pocket dimension at the cost of his right arm. When in his own pocket dimension, Yamantano realised that while he was a pony, his power have been gradually decreased. This made it so that he didn’t get access to all his abilities and thus made him much easier to defeat when he was in his true form. 

Yamantano’s “death” came as a relief for the entire kingdom, but the damage was already done. Princess Celestia was pregnent with the offspring of a member of the Mikaboshi-clan. Yamantano himself knew about this and decided to sit back and watch his future children grow up and potentially destroy Equestria as they was destined to do. But to his suprise, he only got one son; Zweety-Bäl Mikaboshi. Who was abandoned by his mother because of what Yamantano did to her. And if that wasn’t enough, Princess Celestia had been so scarred of the experience that she began to destroy everyone who even dared to threaten her kingdom. Thus in a way destroying Equestria, by turning it into an absolute dictatorship, disguised as the land of harmony. One day Yamantano would return and then, he might be able to reunite with his son.

If he did appear in the show, then he would be animated in a completely different style. A style that is more similar to the titans in Attack on Titan. It would make him look out of place in comparision to the ponies, but that is the point. Since he and the other Mikaboshi’s (besides Zweety-Bäl), where from a totally different dimension.