The Etruscan practice of freely using foreign deities and names to represent their own native pantheon.
While Greeks and Romans prided themselves on the nobility of their own civilization, every indication from the Etruscans shows us the humble opposite - the adoption of foreign aesthetics (ex.: pottery styles from Greece), assimilation of foreign customs (ex.: haruspicy & augury practices from Anatolia), and of course pan-Mediterranean trade. Through large-scale travel across the seas they profited on this classical economy and it was their vast trade in particular that made their amazing cosmopolitan acceptance of the exotic possible, perhaps even necessary. Afterall, good relations naturally protect and cultivate trade.
Their multiculturalism is so all-pervasive in everything they produced or partook that the idea that Etruscans freely assimilated foreign representations and names of gods into their native pantheon is a foregone corollary. However, it does not follow automatically from this that Etruscan religion had no structure or that Etruscans had no native pantheon at all.
Unfortunately many published Etruscologists indulge too often in the propoganda of mystery by using shallow analyses cloaked in pseudoscience, speculation and a perverse celebration of ignorance to shirk the responsibility of providing thorough, educated answers to long-standing questions. Mystery is a good technique for prostituting books and increasing marketability perhaps but overall it disempowers the public through miseducation.
Authors who make claims about the supposed lack of clear number or gender in the Etruscan pantheon, such as the speculations openly pursued by Nancy de Grummond (http://www.utexas.edu/utpress/excerpts/exdegrel.html), are fundamentally groundless since they attempt to legitimize hasty overgeneralizations by way of a deceptive selection of out-of-context quotes from classical authors, some misinterpreted reliefs and paintings, and a handful of "gender-switching" legends some of which are ironically of Greek origin, not Etruscan. Other authors like Larissa Bonfante however do offer opposition to such views and so they should not be touted as unquestionable doctrine.
The varying numbers of gods such as those on a relief of a sarcophagus for Laris Pulena showing "multiple" Vanths and Charuns (pic here) is nothing more than artistic license showing a reduplication of underlyingly one Vanth and one Charun together in a single frame for the sake of symmetry. If we should take it literally, we are at a loss to convincingly explain why both Charun and Vanth appear elsewhere in the singular or why Charun is sometimes represented in a number greater than two. Interpreting any religious art in a literal sense exposes a naive misunderstanding of how religious art around the world abounds in non-literal symbolism and style. It should also be self-evident why Vertumnus, a god of seasons, should lend so naturally to shape-shifting since this is a common allusion to seasonal change. Compare the Navajo goddess Asdzaa Nadleehe. These loose threads are hardly a basis for assuming that all Etruscan deities freely switch gender. Afterall, there is no debate as to what the genders of Tinia, Turan and a host of other mythological figures well-documented in art are.
Instead of a lack of religious structure, Etruscans had a complex one as even expressed by classical authors who are clear about a pantheon with hierarchies and classes of gods such as the Dii Involuti and the Dii Consentes. The Piacenza Liver (a representation of a sheep's liver cast in bronze for use in haruspicy) is deliberately sectioned off with lines indicating the domains of individual gods with specific names. This demonstrates lucidly the established order of Etruscan cosmos which this hepatic microcosm served to represent. What's more, the Piacenza Liver is known to be preceded by a similar Babylonian model in clay made a millenium earlier (see here, here and here) . There is no shred of doubt that their set of divinatory beliefs are related and we also know that the Babylonian pantheon is rich and complex too. Hopefully then we can shed this insidious myth that Etruscan religion and pantheon have no native structure to study. They certainly do. So what then is this structure? Experts draw a blank.
To illustrate a proper application of interpretatio tusca, we might take the god Charun as an example. He was the god pertaining to death whose job it was to take the recently deceased on a journey to the underworld to meet the goddess Vanth. Yet while his name is transparently Greek in origin (derived from Charon, in turn based on Greek chara), some of the motifs that accompany him, such as the use of horses to guide the dead across land (cf. image of sarcophagus relief showing Charun and Vanth guiding the deceased by horse: http://www.ancientworlds.net/aworlds_media/ibase_1/00/05/72/00057208_000.gif) instead of a boat, must be genuine native concepts of the underworld. We then naturally wonder what the true Etruscan name of Charun could possibly be.
Thus begins an illumination on the matter since Charun, being a traveller of sorts, mirrors aspects of Turms, already known to be equated with Greek Hermes and Roman Mercury. The name Turms appears to be native and we even have confirmation that the god had underworld connections (nb. the inscribed epithet Turms Aitas "Turms of Hades"). Furthermore, his foreign counterpart also serves as escort of the dead (http://www.utexas.edu/courses/mythmoore/imagefiles11/images11/orpheus.jpg).
So to represent Etruscan deities fairly, we should speak first and foremost of a native Etruscan god, such as Turms who is then to be understood to have been represented at times as Greek Charon as a symbol of his specific underworld responsibilities. In the case of Charun, perhaps it might then be said that he is an exonym, simply a functional subset or aspect of Turms. This analysis might further apply to other groups of natural equivalencies and deities with known overlapping functions such as Turan-Uni-Thalna, supreme goddess of fertility & motherhood, wife of Tinia. By focusing primarily on native definitions and structure of the pantheon, we can get to understand the Etruscan religion as the Etruscans would have seen it, instead of through the eyes of classical Greeks, Romans and modern-day pedants.