Dolphin

Dolphin is a name used to refer to several species of medium to large sized aquatic mammals on planet Earth. They inhabit all of the world's oceans, with a number of freshwater species also occurring in tropical rivers. They are gregarious predators; fast and agile swimmers, and use sonar to guide themselves underwater. Besides that, they are widely known for their playful and charismatic nature and their advanced intellect.

Dolphin intelligence is sophisticated to the point that numerous scientists speculate that they might have reached actual sapience, just like Humans, as they have exhibited complex languages and display the capacity of abstract thought. They also exhibit patterns of behavior that are unique to a particular group within the same species (i.e. culture). Some groups of dolphins are among the few aquatic species which have been observed to use tools, specifically: holding sponges with their beaks and using them to scrap the seafloor in search for food.

In popular culture
For all these reasons, Dolphins have been depicted as truly sapient in numerous works of fiction:
 * In The Hitchhiker's Guide to the Galaxy series, it is stated that Dolphins are the second most intelligent species on Earth. The Mice are in first place while humans are in third place.
 * As a nod to this portrayal of Dolphins, Derrick J. Wyatt has confirmed that the Omnitrix from the original Ben 10 continuity contains dolphin DNA.
 * In Johnny Mnemonic, one of the main characters that helps the character "Johnny Mnemonic" on his quest was a cybernetic dolphin with the intellect equal to that of a human and had the ability to hack computers and (literal) human memory storage.
 * Larry Niven's Known Space series has Dolphins being officially recognized as sapient in the late 20th century.
 * Dolphins are one of the four member races of the Commonwealth of Planets in Robert J. Sawyer's Starplex, along with Humans, Waldahudin and Ibs.
 * They are recognized as sapient in the Doctor Who universe, particularly the novel Storm Harvest, in which they colonize the planet Coralee along with Humans.
 * In Valérian and Laureline, Ralph the Glapum'tian communicates with orcas, establishing that they are sapient, although a little crude, creatures.