The fey life cycle

Fey have always gotten the short stick in D&D. They aren't well defined, aren't integrated in most settings, and there are very few of them if any. I'm using The Complete Guide to Fey as my primary source in defining fey.

Basically, fey are like the opposite of undead: souls granted a second chance at life. This places them in a state midway between nature spirits and mortal beings. Where nature spirits arise spontaneously as spiritual counterparts of material things, fey are deliberately reincarnated from an existing soul or spirit.

All fey begin life as one of four “base races,” which give rise to the various fey monster entries. Despite the name, they aren't actually races: all fey constitute a single race and their form isn't necessarily hereditary. These base races (and the monsters they give rise to) are:

• Grogan give rise to fey with liminal forms that substitute some humanoid limbs for animal limbs, such as the furry-legged and hoofed satyrs, fauns, korred, forlarren, bird-bodied sirens, etc.
• Puck give rise to smaller sized tricksters and laborers, such as sprites, brownies, redcaps, gremlins, spriggans, pech, etc.
• Shee give rise to the most majestic or terrifying of the fey, including the nymphs, hags, nuckelavees, dullahans, etc.
• Urchin give rise to fey with largely animalistic forms, such as blink dogs, kelpies, kitsune, etc.

How fey reproduce themselves is variable. As The Complete Guide explains it:

• A fey may couple with a mortal, drawing on their partner’s life force to create a new fey. The child’s base race is either that of the fey father or may be chosen by the fey mother. Fey do not necessarily mature into the same kind at their parent. 
• A fey may render the fey nature unto mortals who follow no god or are unclaimed by divinities. This explains why fey may abduct children. 
• A fey woman may invite unclaimed souls (the dead, spirits, elementals, etc) into her womb to be reincarnated as fey, just as if she had coupled with a mortal. 
• Divine beings may become fey if they find themselves near death, and there are certain rare rituals that mortal spellcasters may use to become fey. 
• Any being of requisite power, not necessarily a fey, may create vessels for lost or abandoned souls to inhabit and become fey. They typically have a great measure of power over the fey they create. 

5e expands fey to cover things like hags and blink dogs. This means that a number of traditional monsters may be reclassified as fey in future bestiaries. I will be sure to include additional entries of my own.