How many r's are in the name?
heard as anthocerros
spelled anthoceros
One. anthocerros gives up a letter and becomes Anthoceros.
What the ear did
The stress lands just before it, on THOSS, and English likes to double a consonant after a stressed short vowel. Greek keras has a single r and so does the genus built on it.
A-N-T-H-O-C-E-R-O-S
The full entry for anthoceros covers the name itself.