As you can see in the first two pictures, this Cavalier King Charles puppy was born with what initially might look like a "club foot". We see this fairly frequently (the last two puppies were both Cavaliers, probably coincidentally) and consider this congenital defect to be caused by abnormal positioning in the uterus as the puppy develops. We speculate that perhaps the puppy's leg was extended backwards during gestation or in some other abnormal position which creates a higher degree of tendon laxity than normal, and at birth the puppy has a difficult time retracting it back into what would be a normal position.
Recommended treatment involves physical therapy which can easily be done at home (see the last two pictures) - gently but firmly manipulating the limb back into its normal, flexed position and allowing the puppy to develop greater muscle resistance in the upper leg while pushing against the lower extremity/foot.
Taping the leg into its normal position for a few hours at a time may help as well. Putting the puppy on foam egg carton-like pads as it gets older and starts walking around will help create traction, which in turn will help develop the muscles in the hind legs. Most of these deformities are transient in nature and with continued assistance resolve uneventfully.