The mount rules are for common cases: humanoids riding quadrupedal animals. However, you might allow someone to ride a beast or other type of creature by making a few adjustments. For an intelligent mount (such as a pegasus or unicorn), use the standard rules for mounted combat, but instead of attempting a check to Command an Animal, the rider uses the same number of actions to ask the creature to do what they want. As the GM, you determine whether the creature does as requested and whether Diplomacy checks or the like are needed. It’s recommended you disallow humanoid creatures and most other bipeds as mounts, especially if they are PCs. If you choose to allow this anyway, either the rider or mount should use at least one hand to hold onto the other, and both should spend an action on each of their turns to remain mounted.