Hello. My name is ***** ***** I will try to assist you this morning.
I am very sorry about your plight and, as an animal love myself, I can imagine how distraught you must feel. The good news is that, if the new owners refuse to return the puppy, you may find relief in court. In general, a contract is not enforceable if there was "meeting of the minds" as to an essential term, such as price. In your case, there is no evidence of any mutual agreement - no written contract or anything else that shows that you agreed to give the new owners the puppy for a particular price. While verbal agreements can be enforceable, such cases are typically very fact sensitive - that is, a judge will typically take into consideration other facts, such as the conduct of the parties, possible misunderstandings, or other circumstances.
In your case, since a judge has no written agreement to look at, he/she will listen to the reasons why you want the puppy back. While there are no guarantees, there is a fair chance the judge will find that there was no agreement and that the new owners must return the puppy to you upon repayment (and possibly other nominal costs). You might also suggest that, to the extent that there was any agreement, your terms of sale required the new owners to take good care of the puppy, and that you have come to believe that they are not.
I hope you'll be able to resolve this without resorting to legal action. But I hope this information helps you to understand your options. If you are satisfied, please feel free to rate my answer, as that is the only way I get credit. Meanwhile, good luck to you!