野犬 means 'stray dog' or 'feral dog', often in urban or rural settings, whereas 山犬 implies a wild dog living in the mountains.
Etymology
Compound of 山 (yama, 'mountain') and 犬 (inu, 'dog'), literally 'mountain dog'. The exact historical derivation is uncertain, but the term has been used to refer to the now-extinct Japanese wolf.
