Final answer:
The hippocampus, named after the Greek word for 'seahorse' because of its shape, is the structure within the limbic system responsible for memory formation.
Step-by-step explanation:
The structure within the limbic system that is shaped like a sea horse is known as the hippocampus. The hippocampus plays a crucial role in the formation of memories and is centrally involved in processing emotional responses. Located in the temporal lobe of the brain, this structure's name comes from the Greek word for "seahorse," due to its resemblance to the creature. Studies of patients such as the famous case of HM, who had his hippocampus removed to treat epilepsy, have shown that the hippocampus is essential for forming new memories.