What is the origin of the one-horned rabbit so common in fantasy and isekai stories? Dozens of fantasy and isekai stories have the one-horn-rabbit as a staple beginner's monster.



Some examples:


Re:Monster (day 4)
I Said Make My Abilities Average! (ch. 19)
Kenshi o Mezashite Nyūgaku Shitanoni Mahō Tekisei 9999 Nandesukedo!? (web novel ch. 22)
Nidoume no Jinsei wo Isekai de (ch. 23)
Saikyou no Shokugyou wa Yuusha demo Kenja demo naku Kanteishi (Kari) rashii desu yo? (ch. 2)


What is the origin of this monster, and why is it so common?
It's possibly a reference to Bunicorn, an early-stage recurring enemy in Dragon Quest series.


Bunicorns are rabbits with a single horn, parodying the fabled unicorns. They have gray fur, and the horn is often a yellowish brown shade. [...].
As enemies typically encountered at the start of a game, Bunicorns are not particularly strong, though they typically outweigh the likes of Slimes [...].

The creature first appeared on Dragon Quest III in 1988 (Japanese NES release).
Considering that Dragon Quest is really popular in Japan, I think this is the most possible reason for using it as a beginner's monster, as an alternative to Slime.