I have been mortal, and some part of me is mortal yet. I am full of tears and hunger and the fear of death, though I cannot weep, and I want nothing, and I cannot die. I am not like the others now, for no unicorn was ever born who could regret...
Madness string 21915
In essence, the problem is that, if it walks like a duck and quacks like a duck, it could be a dragon doing a duck impersonation. One may not always want to let dragons into a pond, even if they can impersonate a duck.