My chihuahua was rescued on the streets of San Francisco after an unknown period of time fending for himself. He was a little underweight but not dramatically s

My chihuahua was rescued on the streets of San Francisco after an unknown period of time fending for himself. He was a little underweight but not dramatically so. No signs of previous ownership (no chip, no collar, not neutered) so he was possibly just dumped somewhere at a young age (they estimated he was 1-1.5 yrs when found). He fiercely guards food and will snap at anyone that reaches for him. He hunts insects including flies and cockroaches (he caught at least a few of those on our walks). I don’t think he’d be as effective as a rat terrier, but I suspect he could hunt and kill field mice and maybe small rats.