I don't think we need a fourth option for that.
If you read the text for chimera, it mentions a unicorn as a chimera.
Similarly, you could make a sabretooth lion as a chimera.
The Archelon seems like a massive sea turtle, so taking the Epitome and then increasing it's size should do the trick.
If it is simply a larger version of an existing animal, an Epitome could cover it.
If it's different than existing animals, go Chimera.