The result often is that things are not the way you thought them to be. As it turns out there are several interpretations as far as the appearance of the sphinx goes. There is a very nice wiki article about that here. Once you have decide what tradition of representation to follow hurry up and make the sphinx. Ah, and don't forget to make a riddle for the sphinx. The sphinx here was inspired by an Attica sphinx statue and a Delphi sphinx statue. Here are pictures of them.
Don't be afraid to experiment with less popular ways of representation of a mythical creature. You can also come up with one of you own that is completely different than anything you have seen or heard of. This won't mean it is not a valid representation of the particular mythical creature you are making.
On the other hand it is always good to make something and have people recognize what it is at first glance. So you have to decide on your own what to do.