Why is the s sound written with a c?
heard as anthoseros
spelled anthoceros
Because the letter is a Greek kappa that Latin wrote as c, and English softens c before e. So anthoseros is written Anthoceros.
What the ear did
The horn in the name is keras, with a hard k in Greek. Latin transliteration turned the k into c, and the English reading rule for c before e turned it into an s. Nobody decided the sound should change; two conventions applied one after the other and left a spelling that contradicts its own pronunciation. The same pair of steps produced cyst, cycle, centre and Cerastium, and it catches people out most when the Greek word is one they already know in another form. Keras is the horn of rhinoceros, where the c stands before an o and stays hard. Move it in front of an e and the same letter goes soft. Anthoceros is where the two readings sit four letters apart in one word: the first c is not there at all, and the second one has quietly become an s.
A-N-T-H-O-C-E-R-O-S
The full entry for anthoceros covers the name itself.