Undertale AUs

Undertale AUs are a popular fan concept in the Undertale community, which senok has also made quite a lot of. They depict alternate universes of Undertale's, whether it be them swapping positions, swapping personalities, changing personalities, Sans being the center of everything, or other series characters taking on their positions. Naturally, with the number senok has created, there's been some crossover with the Dorws CYOA.

Firey
Firey originates from an Undertale Roleplay game on Roblox, which served as a hubworld for all kinds of AUs, so naturally, Firey's at least seen a lot of them, at least. At most, Firey is an Undertale AU character, being an original character meant to interact with all of them.

Arms and Head Sans
Arms and Head Sans is, similarly, from an Undertale Roleplay game. He also appears in an unnamed senok-made AU which focuses on characters he has roleplayed as, seriously or not, Arms and Head Sans being one of them. The whole concept of Undertown also reflects this, with the place being overrun by odd senok-made Sans-es.

Background Characters
Quite a few background characters are also from AUs.

Bakseet
Bakseet, with an abundance of many old characters, naturally, has quite a few Undertale AU characters. These include Really Cool and Epic!Sans, Classic Trip!Sans, Classic BK!Underswap Papyrus, and an unnamed background Sans, while also arguably including Friday Night Sansin' Sans, Gun Man, and SiivaGunNer.

Undertown
Undertown also has several AU Sans-es cameoing, including Sans Undertale from TaleSans TaleUndertaleTale Tale and Dans from the 0"Closed Eyes Drawing AU".

Senoktale
The first Undertale AU to feature Dorws characters was Senoktale, an AU centered on characters related to senok. Dorws takes the Sans role, Dark Dorws takes the Papyrus role, Bandana Dee takes the Snowdin Shopkeeper role, Perteli takes the Muffet role, and Dagyvin takes the Gaster role.

Dorwstale
Dorwstale is an Undertale AU entirely centered on the Dorws CYOA. It follows Dorws and Dark Dorws in an Undertale setting, being somewhat darker as a result. In it, all characters are replaced by ones in the CYOA, with Dorws in the Frisk role, Dark Dorws in the Chara role, King Dedede in the Sans role, Bandana Dee in the Papyrus role, and Dagyvin in the Asgore role.

Dorwsrune
Dorwsrune is a Deltarune AU based on the Dorws CYOA which is independent from Dorwstale. It follows the combined stories of Deltarune and the Dorws CYOA, again being an AU where Dorws CYOA characters take over the roles of those in Deltarune. Dorws takes the role of Kris, Dark Dorws takes that of Susie, Bandana Dee takes the role of Ralsei, Dagyvin takes the role of the King, and Dedede takes the role of Jevil. Perteli's role is unknown, as is the Lancer role.