All early medieval Europe art objects I am aware of are either about the Christian subjects or about the history (chronicles etc.). Classical mythology was obviously known in Europe before, see e.g. What did people in 13th century England know about Greek mythology? , but it was not seen in the art (as far as I am aware) until the 15th century.
Then, sometime in the 15th century, we see the emergence of art objects (drawings, tapestries, etc.) showing antique mythology subjects, e.g. this drawing made by Jacopo Bellini in cr. 1420-1470.
When and, if known, why has antique mythology re-emerged in art? Was it because new knowledge has emerged or some restrictions have been abolished?
One of the possibilities is emerging archaeology (Ciriaco de' Pizzicolli, 1391 – 1453/55, has pioneered it in Europe), but there might be other reasons.