Posting a picture would help us tell you exactly if your snaky is an Onyx or not, but Onyx is combo between Black Pastel and Het Red Axanthic. Honestly, it doesn't look like either cinnamon or black pastel. There are big differences like a completely whacked out pattern. Black pastel looks very much like a very dark version of a regular pastel. Black/dark brown instead of yellow. Cinnamons are dark but have way more defined alien heads then an onyx does. A good quality onyx should be black/dark brown with light/bright yellow.

Post a picture and we can let you know.