Ostrich
Ostriches are likely to be placed alone in the order Struthioniformes. They have a number of peculiarities which suggest a separate evolution from the rest of the ratites. They lack a gallbladder. They have three stomachs, and the caecum is  long. Unlike all other living birds, the Ostrich secretes uric acid separately from faeces. Unlike all other birds, who store the uric acid and faeces together, and excrete them together, ostriches store the faeces at the end of their rectum. They also have unique pubic bones that are fused to hold their gut. Unlike most birds the males have a penis, which is retractable and  long. Their palate (bone on the roof of the mouth) differs from other ratites.