According to the tweet below by James Gunn, the answer to who created Rocket Raccoon in the MCU is hidden somewhere in Guardians of the Galaxy Vol. 2.
Which Easter eggs is James Gunn referring to?
Edit 1: While this answer to the question Who created Rocket in the MCU? does provide an anagram as a candidate for the Easter egg, and given that each movie in the GOTG franchise is known for being a treasure trove of Easter eggs, the objective of this question is to inquire if there are other such candidates, and if any of them are more conclusive.
Edit 2: Since Gunn was considering making Annihilus the creator of Rocket and the ultimate antagonist of GOTG Vol. 3 instead of High Evolutionary at one point and also since Gunn had already expressed interest in wanting to play around with Annihilus in the GOTG film franchise as early as 2016, it's possible that the Easter eggs that Gunn is referring to in the tweet are dedicated to Annihilus as opposed to High Evolutionary.