What animal does he fear the most and why?

Ah, you've hit the nail on the head! Doraemon, that all-powerful cat-shaped robot, fears nothing in the world—except for one thing: mice.

That's right! The very idea that a "cat"-shaped robot is terrified of mice is itself a fascinating setup.

Why is he so afraid? There's a heartbreaking story behind it

This isn't about feline instincts to hunt mice. Quite the opposite—his fear stems from deep psychological trauma. This story is detailed in the 1995 movie The Birth of Doraemon (2112: Doraemon Tanjou). Let's break it down:

He Was Once a "Top-Tier" Robot Originally, Doraemon was yellow with two pointy ears. He was a top student at robot school (though a loose screw during manufacturing made him a bit scatterbrained later). Back then, his voice was also higher-pitched and louder.
An Accident During a Nap One day, while Doraemon was napping, a robot mouse from the future chewed up his prized ears! They were left ragged and full of holes.
"Disfigurement" and a Mistake He was taken to a robot hospital for treatment. However, the doctor made an error during the repair and simply cut off his ears entirely, leaving him with the smooth, earless head we recognize today.
The Final Straw To make things worse, his girlfriend at the time (a cat-shaped robot named Doramiko) didn't offer comfort when she saw his earless state. Instead, she laughed at him loudly. This utterly devastated Doraemon.
Sorrow That Changed His Color Heartbroken, Doraemon tried to cheer himself up by drinking some "Spirit Energy Tonic." But he accidentally drank the "Tragedy Tonic" instead. He cried uncontrollably for three days and nights. His tears washed away his yellow paint, revealing the blue primer underneath. His constant crying also ruined his voice, leaving him with the familiar raspy tone we know.

From a Psychological Perspective: It's More Than Fear, It's PTSD

So, you see, for Doraemon, a mouse isn't just an ordinary animal. It's a trauma trigger.

Triggers Painful Memories: The sight of a mouse instantly brings back the excruciating memories of having his ears chewed off, being mocked by his girlfriend, and losing his original appearance and voice. It's an incredibly powerful negative psychological association.
Phobia-Level Reaction: This fear has escalated to the level of a phobia. That's why his reactions to mice are so extreme—fainting, jumping around frantically, or even indiscriminately pulling out weapons like the Earth Destruction Bomb to deal with a tiny mouse. It's a completely uncontrolled stress response, far beyond simple "fear."

To Sum It Up

In short, the core reason Doraemon fears mice is that they were the starting point of all his misfortune. Not only did they change his appearance, but they also inflicted immense psychological trauma.

So, next time you see Doraemon scared out of his wits by a mouse, remember—it's not cowardice. He's reliving his most painful memories.