Phobias are overwhelming and irrational fears that stay with a hero for the rest of his life. Whenever a character is in the presence of his phobia, he subtracts 2 from all his Trait tests as a Minor Hindrance, and 4 if the fear is a Major Phobia.

Phobias shouldn't be too obvious–everyone should be afraid of vampires, for example, so it's not a phobia–it's common sense. Instead, the phobia usually centers on some random element the mind focused on during whatever encounter caused such a fright. Remember, phobias are irrational fears.

Savage Worlds Deluxe Edition p. 32