https://www.novelupdates.com/series/behemoths-pet/
The whole title is...
S-Rank Monster no Behemoth Dakedo, Neko to Machigawarete Elf Musume no Kishi (Pet) Toshite Kurashiteimasu
It's a story of a knight reincarnated into a behemoth but the elf thought he was an elemental cat that could grow 2 meters big and can "Crossbreed"~
Anyway it sounds a lot of fun so~ Kininarimasu!