I really love this art style, it's really charming and works with what you've made. It gets really exciting to see what the next evolution looks like and it kept me going a lot.
Though, it does get a tad bit repetitive near the end but I do really love seeing what new monster will appear. If you want to expand on this, multiple first generation monsters that have different evolution paths would be really cool.
This is a really unique and good submission, good luck for the rest of the voting period!