Horse-headed hippogriffs are not unique to E. Nesbit. E.R. Eddison also had them in his The Worm Ouroboros - and since he illustrated it himself, there's no chance that it was an author-artist miscommunication. (Gaiman may actually have been referencing Eddison rather than Nesbit.)
Jordan
08:45:14 AM Apr 3rd 2011
This is a really not well-defined trope. Most of the examples are either authors who actually did show their work- they just didn't present the most common version of the creature; or else, just artistic license.
