Gligarman here! On my way to the suntouched gym, on one route, I happened to find a cute Marill! Huge Power, Yes! Read much more today about IVS and such. So, decided to look at Marill’s values, and then it happened.
I suspect Marill Man, my nemesis was responsible.
In all seriousness, from what I’ve read on IV’s , Marill cant grow in Attack? Oh, just found out about IV stones
But it is very rare. Based on the insurgence wiki, https://wiki.p-insurgence.com/Poképon, nature change occurred at 1/12 chance. There are 25 natures of Pokemon. So, to get the nature you want, there is 1/300 chance.
1/300 = 0.333%
That’s why, good nature is always preferred first, before wasting IV stones.