Ptolemy gives Capella the nature of Mars and Mercury, and unusually the minor-sounding component dominates the tradition entirely.
The reading that comes down is of Mercury: inquisitiveness, a restless and questioning mind, love of learning and of freedom, eccentricity, honour and wealth obtained through those qualities. The Mars component contributes almost nothing to the standard delineation, which is not martial in tone at all. Compare Betelgeuse, which has the same two planets in the same order in Ptolemy and is read as predominantly martial. Two stars with identical Ptolemaic natures acquire opposite characters in the later tradition.
That divergence is instructive, and it is evidence about the tradition rather than about the stars. If a two-word Ptolemaic assignment can yield a warrior in one case and a scholar in the other, the delineations are not being derived from the assignment. Something else is supplying the content (the constellation figure, the star's colour, its name, accumulated earlier material), and Ptolemy is being cited for authority after the fact.
Capella is one of Agrippa's Behenian fifteen, paired with sapphire and with thyme.
The goat association has a mythological attachment: Capella is frequently identified with Amalthea, the goat that suckled the infant Zeus, whose horn became the cornucopia. That identification is ancient and is a reasonable candidate for the source of the star's association with abundance and good fortune, a more plausible origin for the delineation than the Mars-Mercury pairing, and an illustration of how much of fixed-star tradition is really constellation mythology attached to a bright point within the figure.