Creature with fly speed (no hover) and falling?

Are there any established rules about flying creatures without hover mechanics when it comes to falling?

Example, a pitfall trap opens under an Owlin.

Do they subtract their flight speed from the distance they fall before damage is calculated?  If they used any movement on their previous "turn" does that impact calculation? 

Quote from MobiTraveler >>

Are there any established rules about flying creatures without hover mechanics when it comes to falling?

Example, a pitfall trap opens under an Owlin.

Do they subtract their flight speed from the distance they fall before damage is calculated?  If they used any movement on their previous "turn" does that impact calculation? 

No, none of that is in the rules.

Being able to fly without the ability to hover doesn't have any impact on falling.

Actually that is an optional rule from Xanathar’s, “If you’d like a flying creature to have a better chance of surviving a fall than a non-flying creature does, use this rule: subtract the creature’s current flying speed from the distance it fell before calculating falling damage.”

Yes, Xanathar's has the section that MoribundEidolon mentioned, it's in the Falling section of the book.