One-shots:
(willing prey, safe vore) Predator Safety Club: https://www.furaffinity.net/view/17884172/
(Undertail, safe vore) For Science: https://www.furaffinity.net/view/22339747/
(willing fatal) Dragon's Companion: https://www.furaffinity.net/view/17461724/
(Undertail, fatal) Chocolate Santas: https://www.furaffinity.net/view/25850720/
(Undertail, safe willing vore) A Fishy Story: https://www.furaffinity.net/view/27480393/
(Undertail, safe unwilling vore) Blue Wolf, Red Bunny: https://www.furaffinity.net/view/31840329/

Haku and Mitsu (both willing and unwilling, safe and fatal):
chapter 1: https://www.furaffinity.net/view/22506718/
chapter 2: https://www.furaffinity.net/view/22519166/
chapter 3: https://www.furaffinity.net/view/22528303/
chapter 4: https://www.furaffinity.net/view/22600303/
chapter 5: https://www.furaffinity.net/view/22715354/
chapter 6: https://www.furaffinity.net/view/22794767/
chapter 7: https://www.furaffinity.net/view/22884311/
chapter 8: https://www.furaffinity.net/view/24949838/
chapter 9: https://www.furaffinity.net/view/26268800/
chapter 10: https://www.furaffinity.net/view/26387035/
chapter 11: https://www.furaffinity.net/view/26399194/
chapter 12: https://www.furaffinity.net/view/26415607/
chapter 13: https://www.furaffinity.net/view/26450944/
chapter 14:
chapter 15:
chapter 16:
extra short: Haku gets amnesia: https://www.furaffinity.net/view/15267181/

(Undertail, safe vore) Bluebunny is a Good Friend:
chapter 1: https://www.furaffinity.net/view/26236735/

See also (some overlap between sites):
Archive of Our Own (so much Undertail vore): http://archiveofourown.org/users/wolfbunny
FictionPress (not vore but vore-ish themes): https://www.fictionpress.com/u/968835/mikedono
Wattpad (not vore but vore-ish themes): https://www.wattpad.com/user/blushingskeletons
deviantArt (mostly the same as FA but some stuff that wasn't posted here, or isn't posted here YET): https://lycovore.deviantart.com/gal.....y/51908276/fic