Sheriff Toothpick (Sly Cooper: Thieves in Time) can grow to a massive size when he becomes extremely enraged.
When tainted by Malefor's darkness three years ago, Spyro (Legend of Spyro) transforms under extreme emotional stress, when he was full of grief of his beloved mentor, Ignitus', sacrificial demise.
When the Jinchuuriki (Naruto) become upset enough, they can access their respective Bijuu's power.
Ichigo (Bleach) awakened his hollow powers with anger and determination to protect others.
Saiyans such as Goku (Dragon Ball Z) assume the Super Saiyan form through extreme emotional stress, such as anger.
Steven Universe (Steven Universe) possesses a wide-range of gem-powers all tied to his emotions, such as the ability to control his age, the velocity of his ascent and descent and the ability to summon his shield...
...various abilities inherited from his mother Rose Quartz.
The Man-Thing (Marvel Comics) generates corrosive acid in response to fear, including its own.