Let's take a look at the development of Starcraft 1.
Early Alpha:
As development was beginning on Diablo in 1995, the people at Blizzard Irvine were brainstorming their next game, a project that would attempt the daunting task of surpassing Warcraft II. Bob Fitch, an instrumental programmer of nearly every Blizzard game since Rock 'n Roll Racing, had originally been assigned to work on a game called Shattered Nations, a post-apocalyptic strategy game of warring earth-bound nations, while Warcraft II was still under development. He was later pulled off of Shattered Nations to code Warcraft II's AI, and by the time Warcraft II shipped, Shattered Nations had been cancelled. A new game would be developed instead...a project that would "take Warcraft II and turn it into a space game."
The game would feature three totally different races, as opposed to the similar twin races of Warcraft II. Using the Warcraft II code as a base, Bob assembled a primitive build of StarCraft for E3 1996. Here are some screenshots of the first version of StarCraft ever shown to the public, the Early Alpha Version of 1996:
The Zerg in the Early Alpha were known as the Nightmarish Invaders. Later the Nightmarish Invaders were renamed to the Zurg and even later to the familiar Zerg to avoid legal issues of the Zurg sounding too much like the villain of Disney's Toy Story.
The first image shows us several early Zerg units. The first unit is the Larva, followed by the Drone. The bottom two units on the left side are easily recognized as a Scourge and Mutalisk. The second unit on the right is an Overlord, although it looks a lot like a Guardian at this point. Under that is a Carrier with an Interceptor. In the second screenshot, we see that the Overlord actually has an acid-like attack. In the third image, we see an unevolved Hatchery and a Spire above and to the right. You can spot three Overlords floating around, as well as two of those unidentified units that are in several Early Alpha screenshots. The interface is very Warcraft-like. In the fourth image, we observe that peons were originally flying units that mined minerals from floating asteroids. We also get a glimpse of the first Protoss land unit, perhaps an early build of the Zealot.
you get the picture. When the game went beta the art style still wasn't final. Just saying art direction is decided on before alpha (whihc is incorrect) only means art direction, it doesn't mean anythign is set in stone.
Some beta screenshots, take a look at the Goliaths.
) 
t:
)
