Twisted, broken body of a zombie gnoll no longer resembles a humanoid or a hyena. All that's left is just a pile of rotten flesh and bones filled with malicious urge to kill and consume.
Zombie Gnolls are particularly dangerous enemies because:
Zombie Gnolls appear in the Guts levels (levels 26-30) of the dungeon, as specified in Bestiary.json. They are not found in the earlier Goo level, but rather in the deeper Guts levels.