What is the odd one out and why: Eta, feta, theta or zeta?

While they are all Greek words in origin, feta is a Greek cheese and the others are letters of the Greek alphabet:

In Greek cuisine, Feta (Greek: öÝôá) is a brined curd cheese traditionally made in Greece with ewe's and goat's milk.

Eta is the seventh letter of the Greek alphabet. Theta is the eighth letter of the Greek alphabet, derived from the Phoenician letter Teth. Zeta is the sixth letter of the Greek alphabet.

https://en.wikipedia.org/wiki/Zeta_%28letter%29
https://en.wikipedia.org/wiki/Theta
https://en.wikipedia.org/wiki/Feta_cheese
https://en.wikipedia.org/wiki/Eta_%28letter%29