adult dog; fully-grown dog
Refers to a dog that has reached physical maturity, typically after one year of age. Often used in contrast with 子犬 (puppy).
This dog food is for adult dogs.
Compared to when he was a puppy, he has become calmer as an adult dog.
成犬 is an adult dog, while 子犬 is a young puppy still growing.
Compound of 成 (sei, 'become' or 'mature') and 犬 (ken, 'dog'). The reading is on'yomi, and the word is a straightforward Sino-Japanese compound.