Is there a word for a person with only one head?

The point that a word should make is that it has to be an adequate symbol for what it represents.

This is, in essence, arbitrary and pragmatic and what ever work (as a symbol) will make its way into memories and experience of individuals, then a subculture (jargon) and then culture (spreading from common spoken use to use in periodical publications to the moment these words are added to dictionaries).

It can follow patterns from Greek or Latin, and you will find many example in this and last century — we had to invent many new words to deal with advance of information technology.

If you read up on neologism you will find other ways of constructing new words mentioned — combining existing words (regardless of etymology), abbreviating, rhyming, etc..

• urban dictionary almost exclusively shows neologisms (even fictional)

• Here you can see new words in journals

• You can also look at lists of words added to a dictionary, such as this sample from Merriam-Webster

As you examine these lists you will see progression from less adopted words to more standard words and you might see some patterns that make them accepted into the language.

EDIT:

• There is polycephaly - which refers to general condition of having more then one head
• In case of words prefixed with poly-, the usual counterpart is mono- and so your best bet is monocephaly

After I constructed the word, I looked it up and it exists:

mon·o·ce·phal·ic

Bearing one flower head, as in the scape of a dandelion.

A variant would be monocephalous:

1. Having only one head; in botany, bearing a single capitulum or head.
2. Specifically, having the character of a monocephalus.

or monocephalus:

– noun

1. In teratol., a double monster having only one head but two bodies. Also called syncephalus.

See also syncephalus:

– noun

1. In teratology, a double monster with more or less fusion of the heads: same as monocephalus.

I wish I had come sooner. You see, my good reptilian sister was to put your race in its place, but, alas, she was slain by a sacrilegious buffoon:

As a classicist, I'd begin with what words the Ancients used themselves. The following two are (most probably) the only options in both languages—that is, Lewis & Short and Liddell, Scott, Jones have only these words.

Greek: μονοκέφαλος

• From this, correct formations would be monocephalic, monocephalous, and ?monocephalus. The latter is not used in English, I think, but only in medicine or biology, which use Latin in those cases, not English (they'd be italicized). The first one being the most common and analogous to most other existing -cephal- words, I'd pick monocephalic.

Latin: uniceps

• From this, correct formations would be unicapital and unicipitous. Because neither form exists so far in either the Oxford English Dictionary or Google Ngrams, I'd simply stick with monocephalic to describe your pathetic race.

New words based on a word from another language normally follow the way in which older English words from the same stem/root have been formed. Whenever English first adopts a foreign stem, it is converted into an English stem to make appropriate derivations from. This is to some degree an arbitrary process, and there are often several methods by which it may come about; in most cases, one or more of those become established soon after the word is first adopted, and new words based on the same stem are from then on expected to follow the pattern. If a new stem to be converted is similar to a stem that has already been converted, formation usually proceeds analogously as far as possible.

It is generally preferred that a compound word be made from stems that all come from the same language, though hybrids may be necessary if no single language can provide all parts, or if the language that is used in related words cannot provide them. It should first be checked whether a compound already exists in the language of origin, in which case that should be adopted as a whole; only if such does not exist should one create a new compound.

We don’t have a specific word yet for people with one head, but botanists and other similar scientists have long needed such words. Google Ngrams shows that one-headed, monocephalous and monocephalic have all been used significantly over the years. (Edit: bill weaver points out in comments that single-headed is indeed more frequent than any of these.) Monocephalus also appears, but only very rarely as an English noun: it seems to always be either in scientific names of plants, or else a typo for monocephalous. Unicapital, monocephalon, and various other variations I tried found no hits.

Speculating, I think one-headed would probably dominate everyday use. As a noun-form, one-head is obvious, but could easily become pejorative/derogatory; one-headed person/people would then be the politically correct phrase. Monocephalous would, like many similar classical formations, remain mostly associated with medical usage.