Modding discussion.

Guys, can you tell me why I can't replace the Undead with Dark Knights? More precisely, an error occurs... The unupgraded version looks normal, but instead of the upgraded version, there's just one scythe from the Undead... Ugh... Maybe you can give me a link to a good mod? I would be very grateful!
And also, instead of green dragons, there are fire birds, and the upgrade is to emerald... It's so funny... PLEASE HELP!!!!!!