Its very unusual for a 8 days old baby to get admitted to hospital only for dehydration
in this age group is mostly due to sepsis
or some congenital abnormality like primary lactose intolerance
or congenital chloride diarrhea.
Feeding with bottle itself is a cause of infection.
In newborn initially pass greenish black colored stool called meconium then it is followed by transitional semisolid stool for 4-5 days. Only after that the child starts passing semiformed golden colored stool.
Also during initial few weeks baby passes stool after every feed known as gastrocolic reflex.
So this frequent passage of fat globular motion is most likely physiological in your case, but as it si associated with dehydration consult your doctor and discuss about the possible causes.