I don't recall any “strange platypus-like” creature in D&D, let alone one appropriate for the Aberrant Mind. I can only think of Perry the Platypus from the Phineas and Ferb cartoon. Nevertheless the Abberant Mind's origins describes such a creature:
As a child, you had an imaginary friend that looked like a flumph or a strange platypus-like creature. One day, it gifted you with psionic powers, which have ended up being not so imaginary.
The Flumph is name checked, and I'm aware of that creature. What is the other one?
Best Answer
It's most likely a reference to an unimaginatively named creature called a Thought Eater:
They are described as platypus-like:
(Emphasis mine)
As an aid to this answer, David Coffron says:
So, in other settings this may not be a Thought Eater, but at least in the forgotten realms it is.
References from the wiki: