Turtle Form

Goofy is turned into a sea turtle when he visits the world of Atlantica. In this form, his only familiar part is his head while the rest of his body is that of a turtle.

Kingdom Hearts
In this game, he is able to fight against his opponents using his attached shield.

Kingdom Hearts II
In this game, his fighting skills are disabled as the world is re-design to one that progresses its story via songs instead of battles.

Exception
Although Goofy turns into a sea turtle when he visits Atlantica in most of the games, there has been one exception in Chain of Memories where he retains his true form.

Trivia
In the first game, his fins are yellow, but turn black in the second.

The Turtle and Tortoise Forms were chosen because of Goofy's defensive abilities (the shells reference this). Both forms can also be called "turtles" depending on which kind of English you use, the "Turtle Form" would then be called the "Sea Turtle Form."