At first this sounds like a playful list: a zoo has odd animals such as the emu, zebra, auk, and gnu. But the speaker has seen something stranger — not in a cage, but riding in a limousine. The joke is that privileged, chauffeured people can seem more exotic or absurd than any animal on display.
The mood is wry and amused rather than angry. The simple, almost childlike rhyme sets up a gentle social satire. The speaker feels detached bemusement: the real spectacle is not wildlife, but human vanity and wealth. The poem leaves a knowing smile, as if we are all zoo-goers watching a very strange species pass by.