The Catechism in Questions & Answers

303. What does the prohibition to worship other gods imply?

Any veneration or worship of anything— other than God, the Creator— that could be regarded by human beings as a deity is sin. This includes the worship of living beings, natural phenomena, objects, and real or fictional spiritual beings.
Accordingly it is a violation of the First Commandment to regard statues, animal figurines, stones, amulets, constellations, mountains, trees and fire, storms, etc., for example as gods
The fabrication and worship of the golden calf in the time of the Old Testament also represented a violation of this commandment of God: “So all the people broke off the golden earrings which were in their ears, and brought them to Aaron. And he received the gold from their hand, and he fashioned it with an engraving tool, and made a moulded calf. Then they said, ‘This is your god, O Israel, that brought you out of the land of Egypt!’” (cf. Exodus 32: 3-4).