All these answers are terrible. So boring and stiff :sadvaultboy:
hmm... then: (this applies to all wastelanders)
whenever a child is born into the wasteland, wherever he is, it is a rule that they must be physically fit, like the spartans. so, what do they do? At age 3 they take 15 human children and have them fight to the death against a baby deathclaw. Ususally only one child survives. That child then eats the heart of the deathclaw, gaining its power and strength. from then on, that child cannot remain idle for long periods of time lest the deathclaw inside him overpower the human side. Then they become cannibals and murder people and eat them.
Since deathclaws are rare to find, some people must capture other creatures for the ritual. Some defeat bloatflies, some coyotes, etc.
The Courier's true backstory is that he was from an experimental ritual, where 100 3 year old children battled a legendary deathclaw. After the deaths of 99 children, the courier swung at the deathclaw, taking out the last of it's hp. he then ate its heart, making him one of the strongest people in the world.
These are the creatures each companion ate:
Veronica - A lisbian Power Fisto
Raul - a mexican Mr. Handy
Cass - a cazador that was half-drowned in whiskey
Lily - a loud, purple, nightstalker (yea, super mutants do it too)
Rex - a robobrain (dogs too)
ED-E - the person that created him
Arcade - an eyebot with a broken eye
Boone - a blind deathclaw (...yea)