The first name listed is the translated name, the second is the Japanese original. If only one name is listed, then both are the same. The description below deals with the translated version - if there is no description provided, it's because the translated name is simply a shortened or corrupted form of the original, already has an entry elsewhere on the site (in which case a link will be provided) or is self explanatory. In some cases the English name will be the same as the Japanese name of another monster (not necessarily from the same game) - in these cases, the English name will be a direct link to the entry for the other monster.
100eye (Hectoeyes)
AbysWorm (Abyss Worm)
Adamanti (Adamant Hawksbill)
The adamant hawksbill's name is written as "adamantaimai" in Japanese, "adamanti" is a shortened form of this.
AntLion (Antlion)
B.Dragon (Blue Dragon)
BigBird (Big Bird)
BigHorn (Big Horn)
Blade (Murder Saw)
BstDemon (Beast Demon)
Bucaneer (Buccaneer)
Coctrice (Cockatrice)
Corpse (Bodyguard)
My guess is that this is a mistranslation.
DeadHead (Dead Head)
DesAngel (Goblin Prince)
I looked this up and it turns out that DesAngel is the screen name of someone involved in the emulation community. He probably worked on the translation of FF2 - or knew someone who did - and had his name immortalised.
DkKnight (Black Knight)
Dragon (White Dragon)
DthRider (Death Rider)
DualHead (Dual Head)
DualZomb (Dual Head Zombie)
DvFlower (Devil Flower)
DvMantis (Mantis Devil)
Eagle (Dive Eagle)
ElecFish (Electric Fish)
(Emperor)
F.Gigas (Fire Gigas)
Fiend (Pit Fiend)
Flower (Death Flower)
G.Dragon (Green Dragon)
G.Goblin (Goblin Guard)
"G" is short for either "green" or "guard".
G.Molbor (Great Morbol)
G.Soul (Green Soul)
G.Toad (Gigan Toad)
GChimera (Gorgimera)
Gigas (Hill Gigas)
Golem (Wood Golem)
Gotus (Kottos)
GrOgre (Ogre Chief)
"Gr" is short for green.
I.Gigas (Ice Gigas)
IceLiz (Ice Lizard)
Jelly (Yellow Jelly)
KilrFish (Killer Fish)
Krull (Coeurl)
L.Molbor (Land Morbol)
LandTrtl (Land Turtle)
LegEater (Leg Eater)
M.Sucker (Mega Parasite)
Mage (Magician)
Another word for "magician".
Mallow (Red Marshmallow)
MantaRay (Manta Ray)
Mantis (Killer Mantis)
MantKing (Mantis King)
Molbor (Morbol)
MtGolem (Mithril Golem)
P.Toad (Poison Toad)
Panther (Werepanther)
Pudding (Black Pudding)
Q.Lamia (Queen Lamia)
QueenBee (Queen Bee)
R.Dragon (Red Dragon)
R.Soul (Red Soul)
Ray (Land Ray)
Salmandr (Salamander)
SandWorm (Sandworm)
Sargeant (Sergeant)
Satan (Beelzebub)
SeaDrgn (Sea Dragon)
SeaSnake (Sea Snake)
Shade (Emperor's Curse)
Slime (Green Slime)
Soul (Yellow Soul)
Sphinx (Sphinkmera)
StGolem (Stone Golem)
Stlctite (Stalactite)
Sucker (Parasite)
T.Gigas (Thunder Gigas)
Tusk (Tusk Rhino)
VmpGirl (Vampire Girl)
VmpLady (Vampire Lady)
VmpThorn (Vampire Thorn)
WolfDevl (Wolf Devil)
Worm (Roundworm)
WzOgre (Ogre Mage)
Short for "wizard ogre".
Yeti (Sasquatch)
Another name for the abominable snowman.
Z.Borgan (Zombie Borgan)