Big cleanup, but I should mention where I'm sourcing the Monster names from considering near all of them have never been in a localized game and most haven't had their names localized or display in English.
The source used is the
Monster Hunter Frontier Z website's Monster section, but the English names aren't in an expected place at all. The briefing of each Monster is entirely in Japanese, as usual, but the names of the
image files are in English, and not just as Romaji. Obviously, under any other circumstance, this would not be a reliable source at all, as checking most other websites will just give a generic image file name, often filled with random numbers and letters. If they do have a name they're usually a very quick Romaji or approximation of the name, or spelled/displayed differently for the same name on different parts of the website. In this case, however, it's a different story; any Monster names where the English display (not the localized) has actually been confirmed are totally accurate to these image file names on this site, and importantly these are fully consistent with related Monsters that have similar names or suffixes.
An example of this would be the Monster Espinath (commonly called Espinas), which more recently had it's name confirmed in the
Mega Man x Monster Hunter Frontier Z trailer (both in the thumbnail and briefly in the trailer), with the clear distinction of "th" instead of "s"; but what really adds credibility is that a related Monster, Melaguinath (commonly called Meraginas), also ends in "th" and not "s". Volganos (localized Lavasioth) is named just as it should be, as is Hypnoc (Hynockatrice), and Xiangtien (Shantien) is displayed how it would be in Chinese romanization. Zellaeus (Zerureusu), a relative of Liolaeus (Rathalos), has the same suffix (though admittedly the site excludes the "a", however many appearances of Liolaeus have also excluded the "a", displaying as Lioleus, this because the symbol is supposed to be the Greek æ, so it's still consistent) and Diorex also displays the same suffix as Tigrex.
So even though this is a very obscure and unexpected source, there's enough evidence to suggest that these are the proper ways to display the names in English.