Dog. Also the standard word for someone's lackey or a police informant.

What a Japanese person actually thinks
The loyalty symbolism English speakers attach to dogs doesn't transfer. Japanese reads either the animal, or the insult — being called someone's 犬 is a real slur about servility.

Meaning

Literally: dog
Actually: A dog. Also, in a very live idiom, a stooge: 権力の犬 ('a dog of the powerful'), and 犬 alone for a snitch.

Scores

Naturalness●●●●●

Correct and elementary.

Coolness●●●●●

Four strokes, and the insult reading is close enough to the surface to be a problem.

Watch out

medium犬 for 'informant' or 'lackey' is current, common usage — not archaic slang.

Get this instead

忠 chū
loyalty; devotion to what one serves

Loyalty as a virtue rather than as an animal — and one of the classical bushidō terms.
忠犬 chūken
a faithful dog

If you want the dog *and* the loyalty. Evokes Hachikō, which every Japanese person will get immediately and warmly.

On skin

犬

Orientation: either

Brush.