Eda is called "Owl Mom" by basically the entire fandom.
Calling Amity "Gay Tomato" is pretty popular amongst the fandom.
An example of a character who got a fan nickname before their official reveal, though more of a case of "crew name adopted by the fans": When Luz's voice actress mentioned "Creepy Luz" in an interview, fans began using the term as a hypothetical doppelganger of Luz. In a later AMA, Dana claimed that "Creepy Luz" in that interview was just a joke referring to Luz's behavior in the Season 1 episode "The First Day"... only for "Keeping Up A-fear-ances" to reveal that Creepy Luz does in fact exist in canon, and the name was adopted for her immediately. Although once "Creepy Luz" was properly introduced in "Yesterday's Lie", fans quickly began retracting that nickname. Mainly thanks to many of them finding out that the impostor was Good All Along and absolutely adorable.
Hunter's unnamed cardinal palisman was initially referred to as "Lil' Rascal" since that's what Luz called them in their introductory episode, until it was revealed in a charity livestream and later in "Hollow Mind" that their name is Flapjack.
The unnamed blonde cheerleader with pink streaks in her hair seen in the background of several flashbacks in the first episode is often called Clara in fanfics. This gained traction after Vee's introduction, as people started shipping her and "Clara".
Before his name was revealed to be Caleb, Philip Wittebane's brother was often referred to as "Wittebro". His spouse Evelyn was often called Wittewife.
Shortly after his major appearance in "King's Tide", the Collector became known as "Enzo Gabriel", which is a memetic name in Brazil for spoiled, energetic kids, fitting his personality. While unintentional, the fact it can also be considered a Meaningful Name as wellnote Enzo means "winner" and Gabriel means "God is my strength" while also calling the angel Gabriel to mind certainly helps.
Lilith's Palisman goes unnamed, but most fans have agreed on calling them "Ravioli." Some have also gone with "Artemisia" based on an interview with Cissy Jones or "Branwen" ("White Crow", since it's a white corvid), but after a Post-Hoot livestream following "King's Tide", many seem to have gone with Mike Socks.
The Guard Captain working for Kikimora in "Eclipse Lake" has been named "Captain Wolff" in some fanfics after her voice actress Fryda Wolff.
Hunter is sometimes jokingly referred to as "Gen Z's Zuko", due to their very similar backstories and motivations — both being teenage boys with prominent facial scars, raised by the abusive Big Bad, who start off as villains before slowly being swayed to the heroes' side through positive interactions with them.
Luz, Amity, Gus and Willow together are often referred to as the Hexsquad, as all of them go to and often meet at Hexside. Hunter often gets folded into the Hexsquad post-Heel–Face Turn, though some fans choose to refer to the five of them as "The Hexsquad + Hunter". Vee and the Collector are also sometimes counted in the Hexsquad, although this is rarer.
The child-version of Belos that shows up in "Hollow Mind" is sometimes referred to as "Babybel", a pun referring to a brand of cheese with the same name. Once it's established that Belos is actually Philip Wittebane, fans mostly refer Kid Belos as Pip, a shortened nickname for Philip that reflects Kid Philip's tiny size and immaturity. Some even believe that Pip was a nickname from Caleb when they were children. Should a AU feature a Philip who is genuinely good, expect the nickname Pip to be used as a distinction from the evil Belos.
Belos-possessed Hunter is referred to by fans as Helos, Bunter, or Goop Hunter.
Luz's form when she comes back to life having been given the Titan's powers is called by most fans as Titan Luz.
Hunter's new blujay palisman Waffle that is briefly seen in the Distant Finale of "Watching and Dreaming" has been dubbed by fans as either Pancake or Mordecai.
