Arwen

Arwen is the het ship between Arthur Pendragon and Guinevere from the BBC Merlin and Once Upon A Time fandoms.

The Queen of Air and Darkness/The Witch in the Wood
Guinevere (or rather Guenever) does not appear in this book, but Merlyn mentions her being destined to marry Arthur, using his backsight. Arthur brings this up again in a later conversation, showing that he had spent a decent amount of time thinking about it.

The Ill-Made Knight
The marriage between Arthur and Guenever is entirely off-screen, with Merlin telling Lancelot about it during his knighting ceremony. It is stated within the text that Guenever does not love Arthur, finding him to be too much of an ideal. She is however said to have deeply respected him, despite entering an affair with his best friend. Thanks to Merlin, Arthur was prepared and does not seem to hold a grudge against Guenever.

Merlin
Because Arthur is the King's son while Guinevere (better known as Gwen in the BBC series) is the daughter of a poor blacksmith, their worlds kept the two apart until Merlin came into both of their lives and brought them together through friendship.

Once Upon A Time
Guinevere and Arthur both lived in the village of Camelot in their childhood. Arthur showed Guinevere a tree, in which Merlin was trapped, and said that the man had prophesied that one day Arthur would pull out the sword of Excalibur from a stone and become king. Arthur told Guinevere that he wishes to make her his queen and promised to fill the kingdom with middlemists on her birthday. To prove his point, Arthur gave Guinevere a middlemist flower and lead her to a meadow of middlemists. They stopped when they spotted a sword in a stone, which Guinevere mistook as the one in the prophecy. Arthur then realized the sword belonged to Kay, who bullied Arthur about being an orphaned stable boy. After Kay left, Guinevere encouraged Arthur to follow his heart and go look for Excalibur.

At one point, Arthur and Guinevere got married.

Years after Arthur and Guinevere's childhood interaction, Arthur managed to find Excalibur and became the king of Camelot. Guinevere was happy at his success, but quickly noticed something was wrong. Arthur explained to her that the sword was not whole, as its tip was missing. Arthur dedicated his time to finding the missing part of Excalibur, the Dark One's Dagger, distancing himself from his wife. On her birthday, Guinevere tried to get Arthur to stop his research and dance with her instead. She waited for Arthur for some time until Lancelot came. After discovering the location of the Dark One's Dagger, Arthur rushed to Guinevere and Lancelot. He told them the Dagger was only a day's ride away. Lancelot got ready to leave but Arthur insisted that he would find the Dagger himself while Lancelot stayed to protect Guinevere.

After finding out the dagger's location was Arthur's biggest weakness, Guinevere attempted to get the dagger herself. She hoped that making Excalibur whole would finally make Arthur dedicate more time to her again. During this time, Guinevere got closer to Lancelot. When Arthur and Guinevere encountered again, they had both failed at getting the dagger. Guinevere was attempting to use sand to make Excalibur appear fixed and lied that she had found the dagger. She later came clean and made Arthur choose between his quest or their marriage or else she would follow her heart. Fearing this meant Guinevere would run off with Lancelot, Arthur threw the sand on Guinevere, making their marriage appear fixed. He soon threw sand on the entire village, turning it into a castle.

Arthur and Guinevere remained married for years and Guinevere remained under the sand's effect. Years after the sand was thrown, Arthur and Guinevere met visitors from Storybrooke. A Dark Curse later lead them and everyone else in Camelot to Storybrooke. It is unknown what happened to Guinevere after Arthur died. What is known is that she returned to Camelot along with its other residents.

Fanon
On AO3, the ship has 2955 in its Merlin (TV) tag and only 3 in its Once Upon A Time tag. Just has FanFction.Net has over 2.9k fanfics in its Merlin category, with 317 placed under pairing, while its Once Upon A Time category has 0. Since fans prefer to focus on their BBC Merlin counterparts than their OUAT selves.

Fandom
FAN FICTION
 * Gwen/Arthur (BBC Merlin) tag on FanFiction.Net
 * Gwen/Arthur (BBC Merlin) tag on FanFiction.Net
 * Gwen/Arthur (BBC Merlin) tag on FanFiction.Net

Trivia

 * In the third Tales of Arcadia series arc, Wizards, its version of Guinevere had passed away and the pain of her death awoken a darkness within Arthur. Like in the Once Upon A Time series the two have known each other since they were children, who were betrothed to one another at a young age, and similar to their BBC Merlin counterparts they were both close with Arthur's half sister Morgana.
 * In the Shrek series, Artie (a young Arthur going to "Worchestershire Academy") confesses his crush on popular girl Gwen before he leaves after finding out that he was chosen to be the new king of Far Far Away. Gwen is not impressed at all.