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?