Kemono no Gochisou

On a cold, rainy day, Suu, a carnivore, stumbles upon two tiny furballs abandoned in the storm. The twins, Shaoru and Miku, are gentle herbivores without claws or fangs and don’t eat meat. Determined not to frighten them, Suu embraces a herbivorous lifestyle. As time passes, Shaoru and Miku grow taller than Suu and feel compelled to protect their guardian, who has lost much of his predator-like edge. However, Suu, who has always viewed them as his children, struggles to understand their new feelings...

(Source: Daisy Comics, translated)

Note: Chapter count includes extras.