A Helvan.
Falat was a phase of a Helvan male's life in which his horns grew long and became quite pronounced.
History and specifics[]
A Helvan man in the opposite, dormant phase of their race's biology had much shorter horns.
When landing parties from the Federation starship USS Enterprise visited the planet Helvan in the early 2270s decade, the men of the crew wore prosthetic horns to disguise themselves as Helvans. Due to the difficulty of attaching the larger falat-stage horns, all were surgically altered with the dormant phase horns. (TOS novel: The Prometheus Design)
- The Helvan on the book cover, illustrated by Boris Vallejo, had long horns, but it wasn't specified in any way whether those were dormant stage or falat horns.