the mod "Animania" has hamsters in it along with many other animals, the old hamster mod doesn't seem to have updated for 1.12.2, so i hope this one will be ok

https://minecraft.curseforge.com/projects/animania

you will also need to install the CraftstudioAPI https://www.curseforge.com/minecraft/mc-mods/craftstudio-api

and the correct version of minecraft forge

http://files.minecraftforge.net

if you need any help drop me a message or just reply to this comment, i also wrote a tutorial a while ago that you could also use, however some of the website layouts have changed if you get lost, you can contact me

https://www.planetminecraft.com/blog/minecraft-modding-complete-tutorial/