Quoting that website I linked:

So far, I haven't been able to find these six monsters in the NES version. Either they're missing from the port for some reason (cartridge memory limitations perhaps?), or I've just been unlucky in encountering them:
    Brontosaurus (in terms of dinosaurs, I only encounter Tyrannosaurus and Stegosaurus battles in the "prehistoric" mountain/forest region in the desert area found in map section E1--the Brontosaurus should be located here too, but I've yet to see one).
Brontasaurus (sic) does appear in the list of enemies in the ROM. I don't know if there's a way to make them appear, though...

Man Eating Mare.
I think he must be right in that the Mononoke took the place of this thing because it's not on the list in the ROM.
    Orc Chieftain (I've felled dozens upon dozens of Orcs and Orc Leaders, but have never seen an Orc Chieftain yet). That said, I have noticed that occasionally (regular) Orcs with unusually large HP totals (~60+) can be encountered--it's highly likely, based on stat comparisons with the computer's Orc Chieftain, that these high-powered 'normal' Orcs were supposed to be Orc Chieftains, but they didn't get correctly named. ***
Then a bit farther down the page it says...
NOTE: "Ogre" in the original computer version is "Chieftain Ogre" in the NES port. They're moving up in the world of monster prestige I guess.
The answer to these two naming conundrums becomes perfectly clear when you look at the list of enemies in the ROM. Orc Chieftain is right before Ogre, so it reads ORC CHIEFTAIN OGRE. What happened is that there's a mistake in the program that grabs too much for the Ogre's name and not enough for the Orc Chieftain's name, and thus Ogre became Chieftain Ogre and Orc Chieftain became... just Orc. So there's now two Orc enemies, one with considerably higher HP and strength. And this also means there's an Chieftain Ogre enemy and an Ogre Chief.

 
   Pirate Captain [I've beaten the Antichist ship battle over and over, but he's never part of the crew like he is in the computer version--there are only regular Pirates (and sometimes some common monsters hiding in the back row) on the NES. None of said Pirates has unusually high HP, or other stats, in comparison to the others either].
I think the ship is actually called the Anarchist, and I can't remember if I ever fought a Pirate Captain or not, but he is in the list of enemies in the ROM. He probably shares the same sprites as the regular Pirates if he does exist.

    Water Rat (these should inhabit the various sea/water regions, but I've yet to find one--all the rest of the aquatic critters are accounted for though).
See above post. Not sure, but I think I have seen these somewhere. Could just be my faulty memory, but they are in the list.

    Werephase Mummy.
There is no Werephase Mummy, per se, but Mummy is on the list twice. I don't remember this for a fact, but maybe there really are two different types? And the stronger ones are supposed to be the Werephase Mummy?