Single post in Your Guide to 90%+ Breeding Pairs

I believe the best base rate for exclusives/variant eggs to breed is a 1 in 8 chance, every egg check. Certain ones (like Koroku, or the newest breedable Tournament ones) are much harder than that. Tournament pokes, as the tournament seasons move on, get closer to that 1:8 chance each time, until they are the same as most other exclusives/variants. From the wiki: Exclusive Pokémon Exclusive Pokémon are special Pokémon that are exclusive to PokéFarm, and are their own unique species. Exclusive Pokémon will often be able to produce Eggs of their own species, as well as Eggs of their parent species, e.g. members of the Early Bird Line can produce both Early Bird Natu Eggs, and normal Natu Eggs. When breeding Variant Pokémon, there is a 1 in 8 max chance that an Egg that is produced will be a Variant Species. Note: Exclusives and varients have a lowered breeding rate than a normal Pokémon which makes them harder to breed. Also, breeding "first-generation" Variants is typically easier than breeding their offspring! From the Tournaments Season 23 update: "Season 21 Exclusives can now be bred, however doing so will be very difficult. Check your PokéDex for Egg Group information. The breeding rate of Season 20 Exclusives has been increased."

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Tl:dr - it is at least 8x more difficult to breed exclusive & variant eggs than it is regular ones