How did this happen?
Make a save using the console ( [TAB], then enter the command: forcesave ), then reload.
Maybe use F4 to reset the character. (this returns to the player's house)
Move towards its head (once it is dead). An option will appear to give him a potion. This will shrink it and then it can be swallowed.
You need to have a shrinking potion (the white potion) with you. ( have it in your belt, not in your inventory )
If you don't have one, the statue at the entrance gives some.
The teleportation spell is random. In general, it teleports the target into the stomach, but it can also do anything. It is not possible to predict this. ( random can be modified in custom.ini )
Centaurs aren't finished yet, they don't know how to fight and haven't learned how to swallow prey.
The MC can't swallow alive prey other than with an arrow or the teleportation spell (he can also use enemies' weapons and their swallowing abilities). But he'll learn new skills later. ( maybe with quests )
This location doesn't contain stomach acid, so it takes a long time to kill prey (with a griffin, it takes a very long time).
And Andrice uses a spell that reduces the size of the prey, which is why she can put it inside her.
https://www.saeder.ch/index.php/the-game/items/
The acid potion is missing. ( you need 2 acid mushrooms and a full bottle )