noun
A young dog, typically under one year old. The most common meaning.
子犬が庭で遊んでいる。
A puppy is playing in the garden.
彼女は子犬を飼い始めた。
She started keeping a puppy.
noun
A dog of small breed or size, not necessarily young. Less common than the 'puppy' sense.
あの子犬はもう大人だけど、とても小さい。
That small dog is already an adult, but it's very tiny.
Search-only form; not a standard written spelling.
Compound of 子 (こ, 'child') + 犬 (いぬ, 'dog'). The reading こいぬ is a straightforward combination of the kun'yomi readings of the two kanji.