The way I see them, the guarians are less of a species and more just a general concept, so broad that I feel as though it would be unfair for me to lay claim to it. For that reason, the guarians are an open species!
The guarians were very much designed around being as vague and open to interpretation as possible. As such, there are no true rules as to how you must create your guarian. Instead, there are mostly just guidelines that you're free to follow if you want. In the end, you should mostly just design whatever you find most fun. In general, my guarian designs feature:
Guarians typically have large, pronounced, swooping necks. Characters who have been "guarified" are typically given necks far larger than what they had before
Consider designing your guarian to be faceless, or at the very least, eyeless. Ears are pretty common, while mouths are rarer but still perfectly fine.
Besides this, there are a lot more really subtle aspects that define what it means to be a guarian, but regardless, what really defines a guarian is subjective, and thus, these guidelines are merely guidelines and nothing more. In the end, you should design what you feel is right for your guarian, what you feel actually defines a guarian, and above all, what you find most fun.