Uin was a great whale that served Ulmo, being entitled as the primeval whale.
Originally, the Gnomish term uin ("wave", and later "whale"). Another Gnomish for whale is uimoth ("sheep or shepherd of waves"). Moth stands for "sheep" or "1,000", indicating its original meaning of "flock" (e.g. mothweg means "shepherd").
Uin was also called Gulma's great whale, with Gulma being an earlier name for Ulmo.
Uin is considerate and kind to encourage Rover in nervous and to take two Rovers to trips across the world for a couple of times, and to report Rover's worries to Psamathos shortly after hearing them. He is also charming to wink to Rover.
The whale's physical features are rather unclear although according to the Roverandom, Uin was the oldest of the Right Whales (Eubalaena sp.).
Uin was the oldest and mightiest of all whales, and could have been one of the most momentous of all creatures of Ulmo, as the whale was to pull Ulmo's cart. In the early version of the legendarium, a host of whales centering Uin was also left to carry the island of Tol Eressëa across the Sea of Belegaer to off the coast of Bay of Eldamar near Valinor, the Aman's capital. Uin was also in charge to carry important figures.
Rover described Uin to be 'in every way exceptional' including his capability to rest underwater for long periods. Uin has a titanic strength as his tail lash was described to be 'unmeasured' (not clear whether or not this description counts that Ulmo set the might of Valar in the host of whales) to cause the sea to wrath along with Ulmo's trumpet, and Ossë who was said to be faster and more powerful than Ulmo in terms of raw strength underwater, was left far behind the whales pulling the island with the might of Ulmo, and Ossë couldn't draw the island backward. Uin is also capable of swimming across the world with ease.
Uin was skeptical about whether or not Artaxerxes would be able to lift curses on Rover. He worried that he might "catch it" by taking the two dogs beyond Enchanted Isles and told the dogs to keep it to themselves.
It was unknown whether Uin was actually one of Ainur to have own shapes like the Great Eagles. Some speculate that Uin could have been one of ancient beings present on Arda in likeness with Tom Bombadil.
- Narwhals and sea lions also dragged the cart of Ulmo. 
- Interestingly, Valar means "whales" in Swedish.
- The parcel of Tol Eressëa which was fallen off when Ossë tried to pull back the island westward and Ulmo joined the whales to counter, later became the Isle of Balar, the lands of Ireland on today.
- In the Middle-earth Role Playing, Uin is said to be a spirit of merfolk. Aamumeren Isa, a Maia takes a form of a humpback whale with twice the size of normal adult whales. Especially cetaceans are direct realms of Ulmo, and dolphins are called Ulmodil ("friends of Ulmo").
- Genus Eubalaena means "true whale" or "good whale" and so on, corresponding with the species name "Right Whale" in some ways. Right whales were historically one of the closest of all whales in terms of relating to human histories. Understandably for New Zealand cultures, right whales, being regarded as "(one of) the most important or symbolic whale to New Zealand", can be seen as an important role in the creation myth of the nation, commonly known as the legend of "Whale Rider". Right whales' curious and playful natures sometimes allow humans and kayaks to ride on their back as well.
- ↑ 1.0 1.1 1.2 The History of Middle-earth, Vol. 1: The Book of Lost Tales Part One, Appendix: Names in the Lost Tales – Part I
- ↑ Narwhals on Tolkien Gateway. Retrieved on December 07 2014
- ↑ Drout.C.D.M.. 2006. J.R.R. Tolkien Encyclopedia: Scholarship and Critical Assessment. p.73. Retrieved on December 06 2014