Officially, the only way to force a monster into a scene is through an unexpected event result. That might seem too small a chance for a campaign where dangerous beasts ought to be more likely, such as a sci-fi story exploring new planets, or high fantasy that features large underground complexes populated with oversized fire-breathing reptiles.
What about the cool monsters? There’s a simple answer: In Chapter 7 on encounter scenes, there’s the point that: “The player then rolls randomly or selects results that make sense (from the foes table)…” if monsters are potential foes in the setting, the player can select them as part of a foe scene. Mix and match if you’d like. Keep in mind, too, that conventional foe results can be non-human. Mindless minions might be giant insects or the undead; hybrids/animals and vehicles/mounts could include creatures like giant rats, wolves and eagles, or riding lizards. “Automaton” might be anything big & mindless; it could include plant-based creatures, jellies, jams, puddings, oozes and so forth.
In a recent heroic-level game I’d been playing with street-level supers, big groups of conventional foes weren’t quite cutting it. It was also a pain to generate large groups of foes for encounter scenes. The simple answer was to roll up some supervillains as if they were monsters. Their monstrous conflict and damage levels, plus special abilities, gave the heroes a serious challenge.
The potential downside is that it’s always possible to want a smaller beast but end up rolling a cosmic-level threat. Them’s the breaks; at that point the player will probably want to swap the encounter scene for an encounter scene, or just reconsider how the player character’s group can extricate itself and get away from the threat without taking major casualties.