I'm inclined to agree with Mighty on this one, DT. Not meaning to take anything away from what you said of course. It IS entirely possible that they will try to be more accurate to real-world counterparts in doing a CGI/live action movie (in this one that is slated to be released within the next few years)...but I'm thinking that whatever angles Don Bluth took, in creating and fleshing out the characters for The Secret of NIMH, he did that based upon his own interpretation of the characters. It's very likely the new stuff will be a lot different. And I tend to believe that the new movie probably won't even be called "The Secret of NIMH", but rather may come closer to the original book title, or may even be something entirely different. And the storyline may be much different too. (I'm willing to bet that there won't be any magical amulet, and that Nicodemus won't be a doddering old wizard this time around).

I don't know if a live-action/CGI film will answer any questions regarding the Great Owl's species (unless they use a real owl). The production team will either come up with their own ideas for character designs, or base the designs off of the original film. I think the next best place to look for an answer is on page 57 of the book. There's an illustration that shows the Great Owl, and I think he does resemble a Great Horned owl with the facial markings.

Here's an picture of that illustration (sorry for the large size, I forgot to shrink it down):

https://i.imgur.com/8FmVlkZ.jpg

It occurs to me that clearing up details like this may be the one good thing about a live action/CGI Secret of NIMH.

I think he looks like a tossup between the Great Horned owl and the Great Grey owl. But for my drawing, I think I'll mostly reference the Great Horned owl.

(Not sure why those YouTube links won't hyperlink...sorry about that. I guess you'll have to copy & past those into a separate browser window to listen to them)

I think it may be a tough question to answer with certainty. There is no mention of his species (or any species he is based upon) in any online information I could find (even the Don Bluth and Secret of NIMH Wikia pages), nor in the special features of the movie disk. Given that this movie obviously occurs somewhere in North America (and, since it has the National Institute of Mental Health in it, clearly it occurs in the United States), there are only three possibilities for real-life species which fit the Great Owl's very large size at least: the great horned owl, the great gray owl, and the barred owl (note the first link below).

I'm thinking that what is said on the Secret of NIMH Wikia about the Great Owl is probably correct. It says: "He looks like Nicodemus in many ways like with the eyebrows and hair on his face. He is covered in grey feathers and has yellow glowing eyes." They're correct in that. Very much so. And I believe it's at least possible that Don Bluth intended for the Great Owl to represent the "yang" to Nicodemus' "ying", if you follow. Nicodemus is more of a benevolent and sympathetic character, whereas the Great Owl, while not truly malevolent, is certainly foreboding and intimidating...and not terribly sympathetic. He's a darker aspect of Nicodemus. That does ring true.

However, if you're really trying to pin the owl down to a large species of North American owl (and he certainly is large), then I think the three species I mentioned would best fit the bill. Though his call (when Jeremy and Mrs. Brisby first approach the hole in the tree) would seem to indicate a great horned owl...as their call is a distinct "hoooot hooot". The calls of the other two are different...especially the barred owl's, which is not only haunting, but a bit scary (I've heard them at nights in Virginia, and I can assure you, it's SPOOKY! I've heard other owl species in Virginia too...including the plaintive "hooot" of the great horned owl). Of course, of the three, the great horned owl is noted for preferring to sleep in the hollows of big trees.


http://www.birds-of-north-america.net/owls.html

Call of the great horned owl:

https://www.youtube.com/watch?v=aLTCcUwnIGk

Call of the barred owl:

https://www.youtube.com/watch?v=y5zc-NHIipw

Call of the great gray owl:

https://www.youtube.com/watch?v=m3VpKicsXjE