“With the tail between the legs” is Fido for you.
Put him up against a toy terrier half his size and his tail promptly turns down under his belly.
Maybe he’ll even turn on his back, all four legs limp.
Brave dog!
The attitude of a cowardly or scared dog is so typical that we have long said of a thoroughly cowed, utterly abased, or dejected person that he stands or runs “with his tail between his legs.”