Where the term comes from — and a caveat
"Spirit animal" as used online is a modern blend. Animal symbolism is close to universal — European heraldry, Chinese zodiac, Aesop, Norse kennings all assign character to animals. But the specific phrase, and the idea of being assigned a personal animal guide, borrows from Indigenous North American traditions where totems are inherited through clan and community rather than picked from a quiz.
Plenty of Indigenous writers have objected to the casual use of the term for that reason. Animal symbolism is common property; the framing is not. Worth knowing before describing something as your spirit animal.
Both traditions at once
What is my spirit animal?
Enter a birth date. You will get two answers, because two calendars assign animals by date and they do not agree.
Two calendars, 4.7% agreement
Thirteen periods cannot line up with twelve, so the two systems drift apart across the year and only coincide in one stretch — 5 August to 21 August, when both give the salmon.
Neither is authentically ancient. The Celtic animal wheel was assembled in the twentieth century from Robert Graves' 1948 tree calendar, itself a reconstruction rather than a record. The "Native American zodiac" is a compilation that belongs to no specific nation — the hundreds of distinct peoples grouped under that label had their own unrelated systems, and many had none of this kind at all. We give both because both are what people are searching for, and we say what they are.