Izuku is a merman,curious about humans and always lively for the thrill of adventure. Katsuki is an equestrian, and with his trust worthy companion, Explosion, a seal brown quarter horse,they travel unknown lands.

Katsuki was steering his horse towards the water,letting him walk through to cool off his legs lightly. Katsuki doesn't notice a literal merman swimming near them in the low waters,but Explosion does.