Noko

Tatsu is Noko's pet seahorse.

He has the ability to steal people's memories. With the exception of Nico Robin, he managed to steal the memories of the Straw Hat Pirates. Later he then was forced to return the memories by Sanji but quickly reclaimed them again (with the addition of Nico Robins memories). He was finally defeated by Luffy who accidentely managed to retrieve his memories by trying to ride it. He stole memories because his dream was to become a Sennenryuu.

Noko/Dream
Tatsu controlled Noko into helping him gather memories. Tatsu likely chose Noko because his Nemu Nemu No Mi devil fruit power would be the most useful asset to him. When Noko broke free from the hypnotic grasp he did not seem to retain any memory of Tatsu.

Straw Hat Pirates
Tatsu absorbed each of the Straw Hat Pirates memories and came close to his dream of being a Sennenryuu. However he was beaten by Luffy and will likely despise the pirates for denying him his dream.

Abilities and Powers
Tastu had the ability to absorb others peoples dreams and to take control of other people. However it is shown that if he is hit the memories will start spilling out of his body and back into those that they belonged.

Stealing sleeping peoples dreams: An unnamed ability shown on episode 220 Tatsu was played like a saxophone by Noko which allowed him to steal the memories of those sleeping within earshot of the sound. When 'played' Tastu would make a low, mournful sound. Those with their memories stolen would be able to partially remember the sound and the person who played Tatsu. Whether or not Tatsu can be remembered or not is unknown as he was never mentioned. It is likely that people would pay no attention to the toy-like creature but to the player instead.

Hypnosis: Another unnamed ability used by Tatsu which was used to control both Zoro and Noko. Those affected by the hypnosis may not be able to recall what has happened as shown by Noko when he broke free from the spell.

Last Resort: The closest thing to a name given to this ability, Tatsu creates a blue-white 'explosion' which takes the memories within the raidus of the blast. Unlike the 'Saxophone' technique this takes every memory that those caught have.

Spectral Images: Tatsu has the ability to make the images of people appear within a purple mist. He used this on the Straw Hat Pirates by showing them their loved ones (Shanks, Portugas D. Ace, Kaya, Bellemere, Zeff, Hiluluk and Kuina) and also past enemies (Crocodile, Enel, Arlong and Kage).

Trivia

 * His name comes from Tatsu-no-Otoshigo, sometimes shorten as Tatsu-no-ko, the Japanese word for sea horse. Noko also shares the same name origin.