The Son of God who is fully one with God Himself, the second person of the Trinity, which is one God in three persons.
A ton of the earliest writings in the church were dealing with explaining the Trinity, and nearly every time for the first several hundred years that the Christian church found it important enough to get together and publicly denounce someone as a heretic, it was because of denying some part of that equation (either the Trinity itself, or Jesus as fully God, or Jesus as fully God and one with God the Father). The LDS chucks it out in favor of their own weird "nah, actually everyone's kinda a god, that's what's up with Jesus." A history dive on "begotten, not made" and the Nicene Creed is helpful here, comparing it with LDS doctrine on the nature of Jesus and on the end times.
That makes them heretics to orthodox Christianity (for denying that Jesus is not "one with the Father", all homousia) for a similar reason to why Islam views Christians as heretics (because Islam views the Trinity as an assertion of multiple gods, counter to "Allah is one").
A lot of people see a sort of "X derived from Y" and assume based on lack of any further digging that X and Y are interchangeable, when the very derivation from should call to attention that there was a fork in the road where beliefs diverged. Some of those forks are more divergent from one another than others; Presbyterians and Baptists have more in common than Presbyterians and Catholics, and those two groups themselves have more in common than Catholics and the Eastern Orthodox, and still Catholics and Eastern Orthodox have much more in common than LDS and any Christian denomination you could pick from a hat.