Manda (マンダ)
The concept of Manda is cool, and there's no other monster in the Toho line-up like him, but to successfully pull off a snake-like dragon is obviously a difficult task, which perhaps explains his lacklustre appearances.
Ebirah (エビラ Ebira)
Most Godzilla adversaries are very alien and fantastical in design. King Ghidorah is a three-headed, laser-beam-firing dragon. Hedorah is a giant slab of sludge. Mothra doesn't look like a moth. But Ebirah is a much more earthly monster. He's a giant lobster.