Where are the Centaurs and the Robo Brains? Why were they left out of Fallout 4?

The Robo Brain robots are apparently returning in the Automatron DLC if you read the description I think.

Because Bethesda decided to add other things instead. Robobrains are coming back in a DLC dedicated to robots, though.

Centaurs are because of the nature of Super Mutants in the Commonwealth. Without going into too many details, the Super Mutants aren't dipping everything they find into FEV vats like a deep-fryer at a state fair like they were in Fallout 1 or 3.

Deep-fried Mars bars ohhhhh *homer noise's*


I think Centaurs were always like the super mutants pets whereas now they have mutant hounds.


They were freaky little chappys though, I always got the impression if a Centaur could talk they'd be saying "kiiiiill me!"

Robobrains are coming in Automatron and Mutant Hounds are essentially the Institute Super Mutant's version on the Centaurs in the sense that they act as watch/attack dogs. Also Centaurs wouldn't make sense from a story perspective as the Institute aren't really the type of people who would throw two random creatures or multiple people in a vat of FEV just to see what would happen.

Centaurs need to make a return somehow. Did you see the concept art for them? Wow! Certainly would turn F4 into more of a horror themed game.

I miss the booby trapped baby carriages and the garden gnomes.

I like the monkeys and mannequins as replacements though.

They really don't. Bethesda needs to make new creatures to fill out the world, not continue to use old creatures just because people who clearly don't care about the lore want them.

Centaurs don't make sense for the Commonwealth and you need to accept that.