There are two types of monsters in Onepunch-Man. Naturally occurring ones like the Sea Folk and people that have transformed into them by becoming consumed by their devotion to a particular thing like shadow-boxing a lightcord or just really liking reptiles.
The Monster Association recognizes Garou as somebody well on the road to the latter.
In answer to your question though, his first appearance was in a chapter called The Man Who Wants To Be A Monster.
! I didnt want to spoil anything about ONE's original version, just saying that I find this Garou too much of a softie.