Good or nice would be better translations of snill when describing a dog.
Den isn't accepted here... so I understand correctly, "Jeg har en ny hund. Den er en snill hund" is incorrect, but "Jeg har en ny hund. Den er snill" would be correct?
"Den er en snill hund" would be correct, but it would be better to just say "Den er snill". Why repeat the word 'hund'?
I would not call a dog "kind" in English - "nice" would work, but describing an animal as "kind" just sounds weird.
En god gutt