In fifth-edition Dungeons & Dragons, “monstrosities” are monsters that don’t fit neatly into another category: they’re not beings from other planes, they’re not magically animated objects, they’re not animals or ambulatory plants, they’re not humanoids, they’re not giants or dragons, and they’re not undead. Mostly, they’re magically created beings, or they were at some point far back in time. In some respects, they behave like evolved creatures (the more so the longer they’ve been around), but they may retain oddities of behavior that reflect their unnatural origins.
The first monstrosity I’ll examine is another D&D classic, the owlbear: a creature of grizzly bear–like size and shape (and temperament) with an owl-like head. Applying my usual Str-Dex-Con analysis to it places it in the category of “brute”—high Constitution, extremely high Strength, relatively low (though still above-average) Dexterity—which means its preferred fighting style will be direct assault. Owlbears are dumb (Intelligence 3) and won’t prioritize one target over another. They’re not stealthy, but they’re also hard to fool with stealth, having Perception +3 and advantage on Wisdom (Perception) checks based on sight or smell. They’re pretty fast as well, able to outrun most player characters. Finally, they have standard darkvision, so we can assume that they’re active primarily at night and/or underground. An owlbear encountered in its lair during daytime hours will be sleeping, or at least resting, though the presence of other creatures may wake it up.