'A mouse is an animal. Therefore, a large mouse is a large animal.' This commits which fallacy?

The word 'large' shifts its meaning between the premise and the conclusion.

A mouse being an animal is a claim about kind, not about size.

'Large' for a mouse means large-for-a-mouse, a relative standard.

'Large animal' invokes a much bigger absolute standard of size.

Sliding one term between two senses within an argument is equivocation.