antheceros: which vowel joins the two halves?
heard as antheceros
spelled anthoceros
It is an o: Anthoceros.
What the ear did
Writing e for the linking vowel is common because English uses e for almost every neutral vowel. Greek compounds use o, without exception, whatever the ear reports.
A-N-T-H-O-C-E-R-O-S
The full entry for anthoceros covers the name itself.