Possibly Apis because the context of the story is the Israelites waiting for Moses to come down from the mountain with the Ten Commandments, just after their escape from Egypt, but worshipping bulls was a huge thing in the entire Fertile Crescent area so it could have been anything. Bulls are kinda a big deal at the time period - you see lamassu with human heads and bull bodies guarding lots of temples and palaces in Mesopotamia.
Moses is so pissed about the golden calf because he was literally just talking to god. He leaves them alone after getting them out of Egypt and they immediately get into polytheism.
The actual early Israelite religion was polytheistic, had temple prostitutes, burned cannabis for ritual purpose, and very likely had child sacrifice - probably of the first born.