Final answer:
The term for a fillet beneath the taenia in a Doric entablature is regula. It corresponds to the triglyph above and features decorative guttae.
Step-by-step explanation:
The component of a Doric entablature in question is known as the regula. This element in classical architecture is a small fillet beneath the taenia at the bottom of the entablature, directly above a column. The regula corresponds to a triglyph positioned above it and typically has guttae (small drops) hanging from it. These guttae resemble the head of nails or pegs and serve a decorative purpose, reflecting an aesthetic derived from wooden structures of preceding architectural traditions. The regula and triglyphs are part of the frieze, which is located between the architrave and the cornice in the hierarchical structure of a classical entablature.