Spoiler Alert: WARNING! This page happens to contain a lot of heavy spoilers that are all originated from the Overlord author, Kugane Maruyama and his collection of written works. Anime-only watchers and Manga-only readers, please take some time off to reflect it over for a bit before choosing to expose yourself on reading the information below recklessly.
Using his unknown special skill, Ainz can create Zombie Dragons from the corpses of any type of dragon that died.
According to Suzuki Satoru, a Zombie Dragon would be several, if not dozens of times more powerful than a zombie made from a human corpse. In addition, its increased HP meant that not even a level 100 player like Suzuki is able to kill one in a single 3rd-tier spell.
Ainz considered zombie dragons as a famous example of having strong, physical bodies suitable for the creation of zombies.