There is no known organism that only grows on chocolate agar. Chocolate agar is a type of culture medium used in microbiology, but it's not specific for any particular organism. It's primarily used for the growth of fastidious bacteria, meaning they have complex nutritional needs and require special growth factors.

Here's why chocolate agar is used:

* Hemoglobin: The "chocolate" color comes from the heating process that breaks down red blood cells, releasing hemoglobin and other growth factors.

* Growth factors: Chocolate agar contains a rich source of nutrients like iron, heme, and NAD, crucial for the growth of certain bacteria.

While not exclusive to chocolate agar, some bacteria that grow well on it include:

* Haemophilus influenzae: This bacterium is often found in the respiratory tract and is known for its requirement of the "X" and "V" factors found in chocolate agar.

* Neisseria gonorrhoeae: The bacterium responsible for gonorrhea, also benefits from the growth factors present in chocolate agar.

* Streptococcus pneumoniae: This bacterium can cause pneumonia and requires certain growth factors found in chocolate agar.

However, many other organisms can also grow on chocolate agar, making it a non-specific medium for any particular organism.