Chapter 563
"A monster with a snake head, a monster that seemed to be a mix of a lion and a tiger, and a skeleton-like monster."
The first description of Alberu's dark tiger form actually describes it as a lion and a tiger, a liger, which is a real animal. They also grow larger than either parent and are the largest feline alive.
Wikipedia
They're honkin massive. The novel calls him the Dark Tiger but I think for fanart it might be fun to base his appearance off of the liger or the tigon once in a while too.











