imo there not that rare but to others they might be:) I've had a good amount of twins already. 

I think I only have one set of twins, maybe two if that. I even have some horses with twin boosting equipment and still no such luck. Guess its down heavily to RNG chance since QueenBee has a good set whereas I barely have some.

I might say that twins aren't rare but more uncommon. All of the breeds that i have worked ingame at some point give me twins. If you have discord, you will noticed that in one week some players brags about their twins.