Sontails

Sontails is the slash ship between Sonic the Hedgehog and Miles "Tails" Prower from the Sonic the Hedgehog fandom.

Canon
In most canons, Sonic and Tails are best friends and sidekicks in most adventures, even both show their brother-like relationship. Tails is considered as the closest friend to Sonic and his most trusted companion in the series.

Games
They first time met in Sonic the Hedgehog 2 when Tails saw Sonic running like the wind made Tails impressed, admire him for how cool he was and then began to following him. Sonic eventually stopped and the two became close friends. He began to hang out around Sonic at all times of the day, catching up with him using his two tails.

Sonic and Tails were always there to help each other, their friendship growing until they had a more brotherly relationship. It is often that Tails invents machines for Sonic in his help.

In Sonic Advance 3, they are known as the "Unbreakable Bond" when they team up. In Sonic Rivals 2 it is shown that Sonic trains Tails to learn how to run and fight in their free time. Whenever Sonic has an adventure, he always comes to Tails and asks him to come along and Tails is always ready to help. Tails also used to depend on Sonic a lot, but later realized that he cannot depend on him forever. Sonic is also an idol to Tails, and Tails just wants to be as heroic as he is.

Despite his admiration of him, Tails can sometimes get impatient and annoyed by Sonic's fast-paced behavior and attitude. He downright hates when Sonic ignores him as seen in Sonic Adventure, and he gets annoyed when Sonic leaves him hanging when he runs off or when he is planning to do reckless things without much thinking. Despite facing such cases, though, Tails remains loyal to Sonic and stands by his side to aid him no matter what.

Adventures of Sonic the Hedgehog
They share a very close bond each other whil Tails looks up to Sonic like an older brother, but also sees him as other members of a family, and wishes to run as fast as him.

Their relationship is portrayed with varying degrees, mostly like brothers, but also like parent and child (especially in the Sonic Says segments) with Sonic being more-or-less Tails' legal guardian. Sonic appears to know of this flaw and does his best to protect him, but his protection methods are often the reason why Tails gets kidnapped by Eggman in the first place.

Sonic Boom
In the first episode "The Sidekick" of the TV series, its plot specializes in Sonic and Tails' brotherly relationship.

Fanon
Despite being less popular than Sonamy and Sonadow due to the fandom's mainstream preferences, Sontails has a large fan-following which still extends to this very day.

Sonic and Tails have made it to many "Best Video Game Duo" lists together.

Trivia

 * Sonic and Tails appear together in all the canon continuities except Sonic Underground which is the only canon Tails does not appear in.