The Exorcist - Chapter 14Book 1: : Recollections, (3)

However, he did not know that he was not raising just a harmless pet, but a biological weapon secretly made by the government… a real monster.

As a former librarian, Rand felt that he had been living an ordinary life, the one that he had reclaimed unconsciously every single day.

Recently, he has been a little upset because the pet fish he accidently brought home to raised seems to have undergo some strange changes

Well, it turned into a merman.

- Description from Novelupdates

On his usual way to home Mikado Tsuchio fell into a hole that suddenly opened on the ground. Just like that, Tsuchio falls in a different world, he lands on a Grappler Wyvern and manages to tame it.

Tsuchio decides just like that to live in the other world, the reason for his quick decision: he wants to make a monster girl harem.

Every human on Earth was transformed into a summoned monster and was summoned to another world.