Exonymph

Exonymphs are a bio-engineered race made in (surprise yet again) secret. The very first one ate its maker upon waking up (as a full-sized adult), as most "natural" nymphs consume the first brain they see when they hatch. Upon eating one's head/brain, they gain the memories and knowledge of that brain. They will often develop familiar personalities of the eaten but they still contain their own free will and awareness upon eating a brain. They are not inherently malicious when they do this the first time as it's purely their instincts to consume to know their role.
The existence of exonymphs are not known to the general public and are thought to be nothing less than an urban legend or a cliche horror movie trope.