If Obsidian were to add mammals, reptiles, birds, etc they would have to add a whole new tier.
Generally speaking these animals are far stronger than all insects. Insects have often weak and squishy bodies and their exoskeletons are nothing compared to the scales or tough skin of mammals, reptiles or birds.
If obsidian added one of these animals they would have to add other equally strong animals to maintain balance and none of these could be insects because there virtually 0 insects that could take on a mammal,reptile or any other non-insect creature.
our tiny selves would also realistically have a hard time taking down mammals, reptiles or birds. That’s why I’d suggest sticking to adding more insects and biome to the game like centipedes or caterpillars to preserve balance and a large pool of equally fun builds to use.