What would Ты do to create a realistic robeast?

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Here's a few things I was thinking that might make a realistic take on this trope:
They wouldn't size-shift.

They wouldn't breathe in space(Unless they have enough dna of a creature that can survive without a spacesuit, such as tardigrades[ link ])

They wouldn't use magical powers.

They wouldn't be even part demon

They also have a chance of borrowing elements of a Real Robot( link http://tvtropes.org/pmwiki/pmwiki.php/Main/RealRobotGenre ).