Let's look at the sentence you wrote: "Once an echidna has dug itself into the ground, a hungry predator will". This sentence is a bit incomplete because it ends suddenly. It sounds like it needs more information about what the hungry predator will do.
If you want to complete the sentence, you might say something like: "Once an echidna has dug itself into the ground, a hungry predator will look for it." or "Once an echidna has dug itself into the ground, a hungry predator will check if it can find the echidna."
Grammatically, your sentence is correct up to this point, but it needs to finish the idea for it to make a complete thought.