List of old heresies condemned by the Catholic Church now present in Mormonism and Jehovah's Witnesses The title says it all, I'd like a list of all heresies condemned by the Catholic Church, but note this, it must be before the Protestant Reformation. This time specifically those heresies identified in Mormonism and Jehovah's witnesses.
In 325 A.D. the Church produced the Creed of Nicaea (to distinguish it from the Nicene Creed) in order to exclude one particular heresy doing the rounds back then.  Examination of the Creed of Nicaea shows both the Latter Day Saints and Jehovah’s Witness religions supporting that heresy, in different ways, and to different degrees. I do not intend to do a comparison between the two modern groups (you only ask for a list), simply to state the Creed of Nicaea so that those familiar with LDS and JW doctrines on the Person of Christ will see how they fall foul of the heresy spelled out in that ancient document. I do not think Stack Exchange accepts simple lists, but requires evidence to support answers, hence the details I provide. Because the opening sentence runs on (and on, and on) I quote the first four paragraphs before picking out the various heresies each modern group supports. Here is what the Creed of Nicaea says:


  “We believe in one God, the Father, Almighty, maker of all things
  visible and invisible;
  
  And in one Lord Jesus Christ, the Son of God, begotten of the Father,
  only-begotten, that is, from the substance (ousia) of the Father;
  God from God, Light from Light, Very God from Very God, begotten not
  made, of one substance (homoousios, consubstantial) with the Father,
  through whom all things were made, both in heaven and on earth; who
  for us men and for our salvation came down and was incarnate, was made
  man, suffered, and rose again on the third day, ascended into heaven,
  and is coming to judge the living and the dead;
  
  And in the Holy Spirit.
  
  And those who say: ‘There was a time when he was not’, and: ‘Before he
  was begotten he was not’, and: ‘He came into being from nothing’, or
  those who pretend that the Son of God is ‘Of another substance
  (hypostasis), or essence (ousia) [than the Father] or ‘created’ or ‘alterable’ or ‘mutable’, the catholic and apostolic church places
  under a curse.”


Please note how this ancient document is penned by “the catholic and apostolic church” and how they state that those disagreeing with this Creed are anathema – cursed. That is because they are viewed as supporting the heresy the Creed was designed to expose.

Now, the LDS are anathema (heretical) due to claiming there are many Gods/gods. They teach that the god Elohim fathered his firstborn via a spirit wife, and he was called Jehovah (who had, as a younger brother, Lucifer). This Jehovah became a god himself, and also became the man, Jesus Christ. They teach that there was a time when the Son of God ‘was not’. Hence the LDS violate the first sentence plus paragraph four. That amounts to two particular heresies.

The JWs are anathema (heretical) due to claiming that Jesus was created by Jehovah as the archangel Michael. The JWs disagree on the meaning of ‘only-begotten’, saying that it means he was the first-created creature of Jehovah God, whereas the Creed stresses that ‘begotten’ means the opposite of ‘made’. They are cursed because of disagreeing with paragraph four of the Creed; one particular heresy. They also claim Jesus is a different 'god' to Jehovah God, being 'a god' as per their rendition of John 1:1, which likewise makes them polytheistic, as are the LDS. That is a second heresy.

Please note, this answer does not claim to be an exhaustive list. But it should suffice to give a categoric answer to the question as to why LDS and JW denominations are viewed as heretical by Catholicism (and Protestantism also, let it be noted) today. 

Source: Article ‘What Really Happened at Nicea?’ By James R. White, Christian Research Journal, Spring 1997 pp 30-34