Ah the horses I see where you are going with that. I will point out the remaining legends with the male gender ratios (all myths are labeled genderless from what I can see)
Gen 5: The Forces of Nature (all three of them 100% male)
Gen 6: None lmao
Gen 7: None again lmao
Gen 8: Urishifu (And the pre evolution but that's a whole different question for a different time) (weird 87.5 male 12.5 female split)
Gen 9: The Loyal Three (100%) and Terapagos (50/50)
Of course this requires you to make it all the way to Gen 9.
I will say that even though I did warn about overdoing the futa, I'm not opposed to making the Forces of Nature all futa when the time comes to it not just because the gender ratio logic but if you were to go past gen 5, in gen 8 there is Enamorus, the fourth Force of Nature who is 100% female which could provide a fun writing angle. Don't feel forced to have to put a futa in each chaptor tho write what you feel.