Allow me to give you a screenshot of each book's intro, to help you get a better idea:
MME 1:
View attachment 135755
MME 2
View attachment 135756
Sorry, for MME 3, no such screenshot yet. However, what I can tell you is that:
1. It will continue to build on all 3 official bestiary books and continue to make DMs' lives easier
2. There will be more low/mid level creatures/NPCs with legendary actions (boss types for low/mid level encounters)
3. There will be more classic creatures from previous editions
4. There will be new creatures focusing on creature types what is currently lacking in the official books (one of which I will start sharing in the next 4 weeks)
5. Artwork is 90%+ new/original/not stock art
But yeah, technically, all the creatures are author-created/made up.