This is the whole Zerg lineup in the 2007 reveal. They're not quite arranged easily to identify, but from first appearance on screen from left to right:
Mutalisk, Brutalisk, Queen, Zergling, Corruptor, Ultralisk, Swarm Host, Baneling, Roach, Drone, Hydralisk, Overlord.
I am posting this to help preserve it, because not only will this totally be useful for future concepts but is also quite interesting to see their position on Zerg.
Onto my actual thoughts: I think the Brutalisk was either constantly overlapping with the Roach, or constantly overlapping with the Zergling, which is what resulted in it's removal. It seems more like it was built for harassment but a good support for Roaches, which I believe at the time had a melee weapon which would again cause the Brutalisk to overlap with the Roach. Since it resulted in being the "Hunterling" in the Left 2 Die mod, I'd think they gave this cliff jumping early so all 3 races used the mechanic. It seemed to not work out, however. A shame, as I think it would've worked out today if they stuck with it.
The Swarm Host (Codename: Fatty) people tend to call it's model ugly, but I kinda liked it. It was gross and zerg-like for sure which fit it's theme of infestation. It's two main abilities were Infest and Fungal Scourge. Infest well, Infested either a Gateway/Warpgate or a Barracks and slowly produced Infested Zealots or Marines over the time limit. Fungal Scourge infested a single non-massive unit, causing it to take damage until it died explosively which caused AoE damage once it did. Finally, it also had Dark Swarm. The entire concept of this unit was scrapped and replaced with the Infestor.
The Corruptor's change is clearly obvious. It may have been Anti-Air support, but Zerg themselves were lacking Air-Units in general and thus changing the Corruptor to be an air unit was done.
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This goes without saying, but oh I love the creature designs done on the Zergs.
Oh yeah, they have been really neat basically from the start of the game. But I think they looked best during this part of SC2's life. =p
It all looked super-high detail (Except for the Baneling and possibly Drone), and some of the unit designs were phenomenal.