Left-side 'side-dog' radical - the indexing form of 犬 'dog'. Categorises the character as a four-legged mammal; same radical groups 猫 (cat), 狼 (wolf), 猪 (pig), 猴 (monkey). Here it gives both meaning and category - 狗 IS the prototype dog character.
Right side gives the sound. 句 reads jù in modern Mandarin but had an Old Chinese reading closer to gou, which survives in 狗 gǒu. Pure phonetic borrow; the 'sentence' meaning of 句 is irrelevant. Same phonetic in 够 gòu (enough) and 钩 gōu (hook).
