I know that the Romans had the four-horned saddle and that the Sassanids also had some pretty good saddles, but cavalrymen were fighting as cataphracts long before this. Given the immense weight of their armour and that they were using lances, how did they stay on their horses?
Without wishing to discourage further contributions (I love cataphracts), it's a lot easier to stay in the saddle than you'd think. u/dandan_noodles looks at early cataphracts and Macedonian Companion Cavalry here.