1. You weren't entirely sure how you got lost. Okay, you kinda were, going on a hike by yourself really wasn't a good idea when you didn't know the area too well, but still. Tree branches were smacking you, you were fairly certain every bug in a five kilometre radius had decided you were an all you can eat buffet and for some reason, your Quirk wasn't working. All in all, a hearty cocktail of things to make you miserable. This is when you hear something up ahead that is way too big to be a cute little rabbit. And was that... human speech you heard? Oh hallelujah, you're saved! You rush towards the noise only to pull back the foliage and find something very much not human.
(You can pick what mythological creature crush is)

2. You were never getting on a boat again. This was something you swore to any deity up in the sky listening as you did your best to survive after being in a shipwreck that left you stranded on an island. An uninhibited island too. Well, you assumed it was uninhabited. You hadn't come across anyone just yet. And yet, every day you would find things left for you in plain sight. Little things like shiny rocks or materials to help build your shelter or small bits of food that you only knew were edible cause something, or someone, had already taken a bite out of it. Just as you're considering the fact that you might be going just a little crazy, you finally see them, waiting for you in the middle of your camp when you return from a very unsuccessful hunting mission. And the first thing you notice is that they are very much not human
(Again, feel free to pick)

3. Make one up