I don't think I often see slugs eating the mints but slugs in this area a small. Where do you located? Do you get the finger long slugs and giant snails?
My usual suspects in the mints are spittle bugs and Four lined plant bugs. Someone else mentioned caterpillars.
Can you post photos of the damaged leaves and plant? Some of us might recognize the chew/bite marks and give you a better idea.
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