When you can upgrade a foreign temple to a temple with one of your gods, its not quite the same as the 'adopting a god' that occurs with the Temple of Epona.
In the export_desc_buildings.txt file all the temples are listed. Each temple is classified as a temple of fun, leadership, governing, battle, battleforge, fertility (to name just a few.) If the captured temple is of the same type as one you can build, then you upgrade the temple and it simply changes to the name of your factions equivalent temple.
The temple of Epona (Temple of Horse) has no Roman equivalent, but if one is captured by a Roman faction they can then upgrade it all the way to Pantheon status. Thus, its been 'adopted' by the Romans.
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