Who created him as a character?

Rocket Raccoon was "created" as a character by Bill Mantlo and Keith Griffin according to the Hollywood reporter.

Who created him in the comics?

Rocket Raccoon exists within the Marvel Universe Earth-616 continuity. The character has a complicated in-universe back-story. He first appeared in Marvel Preview #7 (June, 1976). Here is the abridged version.

• Alien humanoids settle the largest planet in the Keystone Quadrant star systems.
• They build a sanitarium to study a variety of mentally-ill patients.
• The aliens build a robotic staff to help maintain the patients.
• The aliens lose their funding and leave having the robots remain behind to take care of the facility.
• The robots chafe caring for their patients and genetically modify animals to take care of the patients so they can leave for other pastures.
• Ranger Rocket Raccoon is the leader of that group of protecting animals.

And for the scientific purists among you: It is likely he is a raccoon-like organism, not necessarily a raccoon from EARTH. But for the sake of the non-scientific, the difference is minimal. Star-Lord recognized him as a raccoon and that is what he calls himself.

Who is his creator in the Cinematic Universe?

No one has been able to find the hidden Easter Egg about rocket yet from the research I've done but I found out (according to Slash Film) James Gunn has commented on the origin of Rocket saying:

“We are going to learn more about where Rocket comes from in the coming sagas. It’s going to be a little different from the comics. We already know a lot about from where he came from. It’s a little bit more horrible than what it is in the comics when you come down to it. We will learn more about that.””

So in the coming films we will learn more about Rocket

According to Nerdist:

That glimpse of Rocket’s backside from the first Guardians of the Galaxy movie already suggested a much darker origin compared to his comic book roots. As originally conceived by Bill Mantlo and Keith Giffen, Rocket lived on a planet called Halfworld alongside other anthropomorphic talking animals like Wal Rus and Blackjack O’Hare. Years later, that fairly peaceful origin was retconned into a new past that established the anthropomorphic animals as experiments at the Halfworld Asylum for the Criminally Insane. Essentially, they were all therapy animals.

According to Screen Rant:

The director previously defended the changes made from the comics, and it’s clear he’s not going to stop making adjustments going forward. The films have already told us that Rocket was experimented on to increase his intelligence and strength, but the reasons have been kept vague. Perhaps in Guardians of the Galaxy Vol. 3, Gunn will show audiences the reasons behind Rocket’s abrasive personality.

After lots of research all I can find are theories about his origin infact FilmTheory have made a great video about it:

It suggests Rocket was a failed Super Soldier experiment and etc. Hopefully this helps!

Who created him as a character?

Rocket Raccoon was "created" as a character by Bill Mantlo and Keith Griffin according to the Hollywood reporter.

Who created him in the comics?

Rocket Raccoon exists within the Marvel Universe Earth-616 continuity. The character has a complicated in-universe back-story. He first appeared in Marvel Preview #7 (June, 1976). Here is the abridged version.

• Alien humanoids settle the largest planet in the Keystone Quadrant star systems.
• They build a sanitarium to study a variety of mentally-ill patients.
• The aliens build a robotic staff to help maintain the patients.
• The aliens lose their funding and leave having the robots remain behind to take care of the facility.
• The robots chafe caring for their patients and genetically modify animals to take care of the patients so they can leave for other pastures.
• Ranger Rocket Raccoon is the leader of that group of protecting animals.

And for the scientific purists among you: It is likely he is a raccoon-like organism, not necessarily a raccoon from EARTH. But for the sake of the non-scientific, the difference is minimal. Star-Lord recognized him as a raccoon and that is what he calls himself.

Who is his creator in the Cinematic Universe?

No one has been able to find the hidden Easter Egg about Rocket yet from the research I've done but I found out (according to Slash Film) James Gunn has commented on the origin of Rocket saying:

“We are going to learn more about where Rocket comes from in the coming sagas. It’s going to be a little different from the comics. We already know a lot about from where he came from. It’s a little bit more horrible than what it is in the comics when you come down to it. We will learn more about that.””

So in the coming films we will learn more about Rocket

According to Nerdist:

That glimpse of Rocket’s backside from the first Guardians of the Galaxy movie already suggested a much darker origin compared to his comic book roots. As originally conceived by Bill Mantlo and Keith Giffen, Rocket lived on a planet called Halfworld alongside other anthropomorphic talking animals like Wal Rus and Blackjack O’Hare. Years later, that fairly peaceful origin was retconned into a new past that established the anthropomorphic animals as experiments at the Halfworld Asylum for the Criminally Insane. Essentially, they were all therapy animals.

According to Screen Rant:

The director previously defended the changes made from the comics, and it’s clear he’s not going to stop making adjustments going forward. The films have already told us that Rocket was experimented on to increase his intelligence and strength, but the reasons have been kept vague. Perhaps in Guardians of the Galaxy Vol. 3, Gunn will show audiences the reasons behind Rocket’s abrasive personality.

After lots of research all I can find are theories about his origin infact FilmTheory have made a great video about it:

It suggests Rocket was a failed Super Soldier experiment and etc.