We know that the location Kaido and King met was Punk Hazard. The machine King was bound to when Kaido found him appears to be an MRI or CT Scanner, meaning at that particularly time they were not experimenting on him (in the sense that people think of when they think “scientific experiments”) and were, more accurately, simply studying his body.
The moment Kaido and King met was around 30 years ago, long before the island was blown up by Caesar. Of course, Vegapunk was most definitely working there at the time, in fact, this was probably around the time when Vegapunk was trying to create the Artificial Devil Fruit, which is most likely based on Kaido’s “Lineage Factor” (and was probably why Kaido was there on Punk Hazard in the first place).
Now… they had been studying Kaido, and were currently studying King, which leads me to the interesting thought.
Kaido and Momonosuke’s Dragon Forms are practically identical, though Kaido is Azure and Momonosuke is Pink. However, there is one major difference: their hair.
Kaido’s hair as a Dragon is black just like his human form (even in the anime), while Momonosuke’s hair (which is also black) changes colour (in the manga, it turns grey, we’ll have to wait a while to get the actual colours). However, the differing colour shows Momonosuke’s hair seems to be depicted as taking on a flame-like quality.
My theory is that the Artificial Zoan Momonosuke ate isn’t 100% based on Kaido’s Dragon fruit (though it is mostly based on it), but also incorporates Lunarian DNA to patch up genetic gaps Vegapunk couldn’t fill (due to incomplete study on Kaido). Naturally, this would give Momonosuke a different (expanded) power set to Kaido, more specifically enhanced durability and control over fire.
The hybrid nature of the Artificial Devil Fruit might even be why Vegapunk deemed it a failure (because it’s not really a recreation of the Seiryu Devil Fruit and is something new entirely).
Theory by Pidmik (https://www.reddit.com/user/Pidmik)