The creatures on the Undead cards look nothing like the actual Undead...
...because several cycles ago, the battling Undead grew so powerful that they caused a global cataclysm, and the winner back then, Human Undead, demanded that all Undeads' powers be nerfed down till they were all humanoid-sized. It's the very reason none of the Undead animals are dinosaurs.
- Jossed, we had a flashback of the original battle, and they were the same.
The reason that all of the undead are humanoid, is because the human undead won the previous battle.
The undead take on a form roughly similar to the current dominant species on earth. If, for example, the stag beetle undead had won, we would be watching a bunch of beetle-monsters duking it out instead. Also, as with the above theory, the images depicted on the undead cards are likely their true forms.
- Jossed, they were humanoid during the battle fight original.
All because the creator does not like fugly animals, he revised his version with mostly Bishounen Line
elements and beat plagerism in the process!