Aries Design: Truly Aries, or actually based on Capricorn?

[Rafal]

Based on the manuscripts i've come across so far, capricorn was more commonly depicted as a mountain goat than as a sea-goat

I would say that in medieval times sea-goat was still more common.

Sea-goat is the original, traditional way how Capricorn was presented. It comes yet to Babylonia and is linked to Babylonian god Enki:

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The planet Mercury was thought to be Enki, but Enki was also shown as a constellation of a man pouring water. He was also associated with the goat-fish, a mythological creature with the head of a goat but the body of a fish. Enki became the Western zodiac sign of Aquarius, while his goat-fish became the sign of Capricorn 

If I remember correctly, Capricorn presented as normal goat is a German invention. German illustrators were more "brave" than other ones and often changed details of Zodiac imagery. They changed Capricorn to normal goat and Saggitarius from centaur to a walking man. These details are one of the strongest hints that Voynich Manuscript has some Germanic background.
In other sources like French or Italian Capricorn remained mainly a sea-goat.

Maybe you encounter normal goat more often because your "dataset" is skewed, having disproportional number of German manuscripts?