Left mouth-radical - the indexing radical. Marks 吼 as a sound-from-the-mouth verb, putting it alongside 喊 (shout), 叫 (yell), 唱 (sing). The mouth here is wide open, releasing a roar.
Right 孔 supplies the sound (kǒng → hǒu, an irregular but established phonetic match in the same velar series). Faint semantic resonance: 孔 means 'opening, aperture' - and a roar is what comes out of a wide-open mouth. Hand-mnemonic: an open hole bellowing.
