Chevalier and Valhun, its great to see observations on motif and I'm with you guys, in always trying to seek potential symbolism imbued in weapons.
Although it is tempting to think so, the eye shaped device on the hilt seems to have held some item such as mother of pearl or bone insert that probably aligned with the motif. The evil eye concept prevailed on North African and many Arabian weapons, but not on Spanish colonial.
The inside langet for carrying these often in a sash rather than scabbard seem characteristic on these shellguard examples of espada ancha (which is actually rather a misnomer as technically it describes a wide bladed heavy sword commonly used by Soldados de Cuero on Mexican frontier). It is interesting to note the semi circles applied overall in the hilt motif which recall the 'sickle marks' often widely seen on trade blades.
Valjhun, the distinct nocked grip on Moroccan sa'if (nimcha) hilts has long been considered in some degree zoomorphic, much like the hilt of the flyssa, and the trilobate hilt of the karabela....but I dont think there has ever been an accord on what creatures these overstylized hilts represent.
I had never thought of a horsehead, and would very much like to know of references that specify that identity, it would be fantastic to have that resolved!
Amother really unusual thing on the hilt of this example is the very clearly represented bird on the shell, and I'd like to discover more on this. Bird heads of this form in degree are seen on hilts in specific areas in Mexico, and may have to do with the eagle and snake on folklore and legend there, though the head is admittedly not very eagle like. Regardless this might establish an interesting association between Mexico and S.America.