One reason is the contrast. Sans is usually seen as a cool and calm character in the original Undertale. So when we see him angry in the fanfic, it's a big change. It can create a lot of tension in the story. Plus, the fanfic writers can use his anger to drive the plot forward in unique ways. They can explore how his anger affects the world around him, the other characters, and even his own self - identity. This exploration of new aspects of Sans' character through his anger is what makes 'angry sans fanfic' so appealing.
One popular 'angry sans fanfic' is 'The Raging Battle of Sans'. It really shows an intense side of Sans. Another is 'Sans' Fury Unleashed' which delves deep into his anger and the reasons behind it. There's also 'Angry Sans: The Outburst' that has some interesting plot twists related to his anger.
One characteristic could be the unique blend of the horror and lust elements from the two 'sans' characters. It might play with the contrast between the spooky or menacing nature of horror sans and the more sensuous or alluring aspects of lust sans.
A popular Ink Sans x Error Sans fanfic is 'Shades of Their World'. In this fanfic, the author does an excellent job of exploring the different aspects of their personalities. Ink's chaotic but positive energy is contrasted with Error's more brooding and negative nature. It makes for a really engaging read as you watch their relationship develop. There's also 'Ink's Influence on Error'. This one shows how Ink's presence starts to change Error in unexpected ways. It's a short but sweet fanfic that leaves a lasting impression.
One popular sans x sans fanfic could be those that explore their unique friendship in different AUs (Alternate Universes). For example, some might focus on how they interact during a time of crisis in the Underground. Another type could be the ones that delve into their shared experiences and how they support each other through hardships. There are also fanfics that might take a more comical approach, showing their humorous banter and silly antics together.
A 'baby sans fanfic' is a type of fan - created story that features Sans as a baby - like character. It often involves creative interpretations of the Undertale universe with this unique take on Sans.
These fanfics can be a creative outlet for some writers who want to explore the darker side of the Undertale universe. But they also face a lot of criticism. The idea of a beloved character like Sans being portrayed as a cannibal goes against the general positive perception of the character. And it may not be suitable for younger or more sensitive audiences.
The popularity might be due to the fact that it allows for a lot of exploration of the Undertale lore. Error Sans is often associated with glitching and breaking things in the game's world, while UnderSwap Sans comes from a swapped - personality version of the original. A fanfic can play around with how these two different concepts interact. Maybe it shows how Error Sans reacts to the more positive and light - hearted UnderSwap Sans, or vice versa. It gives fans a chance to see different sides of these characters that they might not get to see in the original game.
To write a great Error Sans x UnderSwap Sans fanfic, start with a strong concept. Maybe it's a story about how UnderSwap Sans tries to 'fix' Error Sans in some way. Research the existing fan works and the official Undertale lore to get inspiration. When it comes to the writing itself, build up the relationship between the two characters gradually. Show how they initially react to each other, and then how their relationship evolves over time. Use vivid descriptions for their actions and the settings around them. For example, describe how Error Sans' glitches affect the environment they are in, and how UnderSwap Sans' positive energy can sometimes counteract that. Also, make sure the pacing of the story is right, not too slow or too fast.
I'm not aware of any specific 'horror sans x lust sans fanfic' to recommend. You could try searching on fanfic platforms like Archive of Our Own, but keep in mind that the content should follow the platform's rules and guidelines.