The flyby trait allows a creature to leave a creatures reach without provoking opportunity attacks. There are a number of ways you can use this ability to make combat more interesting. Here’s the text from the owl stat block:
Flyby. The owl doesn’t provoke opportunity attacks when it flies out of an enemy’s reach.D&D5e Basic Rules
Flyby has a couple of uses. It is useful for mounts, allowing the rider to move in and out of reach without fear. It makes owls useful as familiars. They can deliver touch spells and flee to safety. It is used to devastating effect by phoenixes. These creatures deal damage when they enter another creature’s space. The flyby ability lets them fly through multiple foes in a single turn without risking a bunch of attacks.
Flyby can be especially devastating when the players don’t have a means of flying. It allows the creature to attack and then return to a safe distance. Most dragons take a huge risk getting into melee range with players. A solar dragon with flyby can spend the time it takes to recharge its breath weapon swooping past its enemies for melee attacks.
Creatures With Flyby:
Yeah those are some great monsters, but what’s better than monsters that can swallow a player whole?