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It was Satan
”I acknowledge, indeed, that from this place alone nothing more can be collected than that men were deceived by the serpent. But the testimonies of Scripture are sufficiently numerous, in which it is plainly asserted that the serpent was only the mouth of the devil; for not the serpent but the devil is declared to be the father of lies,' the fabricator of imposture, and the author of death. The question, however, is not yet solved, why Moses has kept back the name of Satan. I willingly subscribe to the opinion of those who maintain that the Holy Spirit then purposely used obscure figures, because it was fitting that full and clear light should be reserved for the kingdom of Christ. In the meantime, the prophets prove that they were well acquainted with the meaning of Moses, when, in different places, they cast the blame of our ruin upon the devil. (Calvin, John. The Complete Biblical Commentary Collection of John Calvin. Kindle location 2840)”.
This view is consistent among Calvinists.