Does find familiar transform a creature of a certain type into a familiar?

@ChrisPerkinsDnD Does find familiar transform a creature of a certain type into a familiar or is it a type of formless spirit given form?

— Kendall Landers (@knlndrs) August 5, 2016

The second thing, unless the DM decides otherwise. #WOTCstaff https://t.co/TR1AxSE3Qs

— (((Chris Perkins))) (@ChrisPerkinsDnD) August 5, 2016