One answer I learned, heavily steeped in aggadah, was that there was no intent to build a calf. One of the things thrown into the molten gold was something that had the words "aleh shor" written on it, which was previously used to raise Joseph's bones from the Nile (as Joseph was referred to as an ox, or compared to, I forget) so a cow form arose. That is why Aaron in Ex 32:24 says that the calf emerged, not that it was designed as a calf.
For more on this explanation, read http://www.jewishvirtuallibrary.org/jsource/judaica/ejud_0002_0007_0_07514.html and scroll down to "In the Aggadah."
I also learned that the calf was used because it was reminiscent of the Egyptian deity-form so the people would accept it.
Rashi (in addition to citing the above Aggadah), on 32:4 says that Egyptian sorcerers created the shape through sorcery:
As soon as they had cast it into the fire of the crucible, the sorcerers of the mixed multitude who had gone up with them from Egypt came and made it with sorcery. [See commentary on Exod. 12:38.] Others say that Micah was there, who had emerged from the layer of the building where he had been crushed in Egypt. (Sanh. 101b). In his hand was a plate upon which Moses had inscribed “Ascend, O ox; ascend, O ox,” to [miraculously] bring up Joseph’s coffin from the Nile. They cast it [the plate] into the crucible, and the calf emerged. -[from Midrash Tanchuma 19]