The Animagus is a shapeshifting nature-oriented wizard with a particular affinity for one specific type of animal. Unlike a druid who intuits the ways of nature and its many forms, the Animagus carefully studies nature with a particular devotion to one or two forms. The Animagus' animal form grows and changes with time to reflect their specialty. 

Shapeshifter

At 2nd level, you choose a Beast with less than 1CR (from PHB Appendix D or MM Appendix A) – this is your preferred form. You may transform to and from this form as an action at will. You retain your Intelligence Wisdom and Charisma scores and saving throws, but are otherwise an ordinary beast of that type, unable to speak or cast spells in this form. In both forms, you understand and can be understood by beasts, although their limited intelligence can be a hindrance on what they have to say. If you are rendered unconscious in animal form you revert to your natural form. When you gain an Ability Score Increase, increase your animal ability scores (StrDexCon) in addition to your humanoid form.

Additionally, as you learn new spells, you may choose from the Druid spell-list, recording them in your spell-book and preparing them as you would a Wizard spell.

Best of Both Beasts

Beginning at 6th level when you resume your humanoid form, your highest ability score in animal form is retained, likely granting a higher Strength, Dexterity or Constitution score. If the highest scores are identical, you retain both.

Your experiences in animal form have also given you insight into the minds of beasts and nature. You gain proficiency in Nature, Survival and Animal Handling if you were not already proficient.

Primal Form

At 10th level when you transform, you may assume a giant or dire version of your animal type (If there is no Giant or Dire version described in the PHB, MM or other sources, choose a similar creature with a CR of 1-2) and retain your hit points in animal form. Your highest ability score in this form is not retained when you resume your humanoid form, unless you have added to it via your Ability Score Improvement.

Speaking as a Beast

At 14th level you perform a type of awakening ritual, you have learned to speak and cast spells in your animal form, so long as you are holding or wearing an arcane focus.