There is a pair of fantastic Harpies created by Jim Henson's Creature Shop. Though you can tell they're CG, the artistry is there. They interact well with the actors and they look really creepy.
The biggest CG monster in the film is a giant dragon that guards the Golden Fleece. I love the texture of its skin and it looks great when the light hits its eyes. Again, you can tell it's CG but for a television movie in 2000 it's really good.
One of the weirder creatures in this film is a metal, mechanized bull that breathes fire. It's a really strange creature to begin with and it has to interact a great deal with Jason London as Jason who rides it and fights it in a lengthy sequence. I don't think the bull is especially successfully realized but it's clear that a lot of work went into it.
There's a briefly seen centaur in the film who's played by Hugh Quarshie. He's seen only in the dark so it's difficult to tell but I think the horse portion of his body is CG and Quarshie's torso has been composited on top.
Possibly my favourite creature from this film is Poseidon. He's created with interesting prosthetics on an uncredited actor. The Argo encounters him as a huge rocky island in the sea and then he awakes and stands up, dwarfing the ship. It's a really cool idea. I've never seen Poseidon portrayed like this before and I really like that it was created with practical effects rather than CG.
Overall, the creatures in this film are not as memorable as those in the 1963 Ray Harryhausen film but they seem to fit the world the filmmakers were trying to create and are pretty successfully realized.