狗，口语话，犬，书面化

Both of them refer to "dog", the “犬” is more often used as written language, while the "狗" is more often used as oral language. Actually, the "犬" was first used by ancient Chinese to refer to dog. People use "狗" more often now.

They're the same. 狗 is verbal. 犬 is in writing. But in specific idioms, they aren't not exchangeable. Such as 雞犬升天、豬狗不如

Basically we have a conversation with someone, we use 狗 . E.g: The dog is so cute! 这只狗好可爱！ and 犬。。We use it in writing and it is more for formal writing.

Agree with Sijing and the latter one is more normally seen in ancient Chinese