The Fire Mouse Pokemon, Cyndaquil, is the Fire-type starter of the Johto region, and as of Pokemon Scarlet & Violet's Indigo Disk DLC, you'll be able to find Cyndaquil in the wild. However, it will only appear if you contribute Blueberry Points (BP) to one of the League Clubs in Terarium. You'll want to catch and evolve Cyndaquil to unlock their Pokedex entries, but it's ultimately your decision whether to use it in battle or not for the remainder of your Pokemon Scarlet/Violet campaign.