Thing about the monsters, there weren't really very many to cover outside of what had been covered in the d20Modern core book and the Menace Manual.
Here are the creatures in the D*M core book:
Armodont - (Uncovered, an odd Grey-dog)
Bering Demon - (D20M replaces with many fiends)
Ekimmu - (d20M replaces with D&D vampires)
Elohim - (MM)
Etoile - (MM)
Ghost - (in the MM or UA, I forget)
Ghoul - ((UA?))
Grey - (MM, d20D*M)
Grey Scout Ship - (MM)
Homunculus - (UA)
Kinori - (Uncovered, a lizardfolk from Egypt)
Kinori, Blackscale - (Uncovered, an advanced Kinori type)
Luciferan - (MM)
Man In Black - (Uncovered? Are the Ghyardi in MM?)
Mole Person - (Uncovered, possibly Mongrelmen)
Mothman - (Uncovered, winged Greys with native-styled magic)
Mummy - (d20Modern, which is a D&D style mummy to this zombie-mummy)
Sandman - (MM, Sand Slave)
Sasquatch - (MM, d20D*M)
Yeti - (Sasquatch, Cold subtype)
The Kinori and the Mothman are the two "big" ones left uncovered ... both had a few major plays in the original game, and the Mothmen had a large part in the biggest published adventure for the game. I, myself, didn't really like the "flavor" of either of them.
The Kinori were a step outside of modern mythology ... I don't see any myths that mention the Egyptians were propped up by D&D Lizardfolk.
Likewise, the Mothmen didn't float my raft. I prefer a more otherworldly Mothman, along the lines of The Mothman Prophecies. They're an interesting take on things, but it just seemed like a real step to one side. "All that weird stuff was unconnected, and the moth-man ... was a whole race of native-american grey aliens with WINGS that nobody noticed."
The Mole People appeared in one of the novels. Really they're just ... people ... who live underground. Some of them, though, have mutated and can regenerate and are immune to disease. It's a homeless-people civilization.
The Man In Black was one take on what they say are many types of MiB. Various governmental conspiracies send MiBs out to see people. The game-stat MiB is an unexplained psychic/shadow-magic using guy that conforms to some of the more out-there MiB reports. They were expanded into their own race, the Ghyardi, for Star*Drive and I think they're covered in the MM. My copy is elsewhere.
The Armodant, I don't think anybody will miss. It's just a bizzare Grey tracking animal ... like a hairless dog with a Grey-like face. Never heard of them in mythology, don't remember them being used in any published adventures or books. I never used them.
Replacing the Ekkimu with D&D Vampires ... ehhh ... the Ekkimu was, I thought, an interesting take on vampires. An alien symbiote that takes the form of a liquid that causes a few changes to the human body to make it vampire-like, and can place the person into a suspended animation so they live longer. The vampire-slime can creep out into a new host body. I thought the concept was cool, so I've since done my own take on the theme.
--fje