Fairies occur magically rather than naturally, I would think. But as a race they completely blur the lines of everything and make it confusing to think about for everyone, so must just accept that they exist and move on.

Slipping pants on a fairy mid flight seems like a fun event in a festival, like pin the tail on the donkey. Only rather than pants I think it would be better to have it be like a medallion, anklet, etc., or simply catching them. Of course it would be close to impossible to win (without bribing the fairy) but you'd get a prize. 

For more information on their magic, fairy magic is mainly powerful over time for effects like magical protection rather then short bursts of powerful magic such as fire shooting.