None of the ferret foods sold at Walmart are nutionally sound for ferrets neither are 99% of cat foods. Some kitten foods are good though. Walmart's Maximum Nutrition kitten formula is quite good, I use it as part of my mix. Eukanuba kitten and Nutro Max kitten are also ok. Ferrets really should be fed a good ferret food though. Zupreem, 8in1 Ultimate (no other formula), Totally Ferret, and Mazuri are all good foods but you'll have to get them from a petstore. Marshall's ferret food is ok but it's fish based and makes ferrets and their feces smell stronger than the chicken based foods.
Pelleted litter is best for ferrets. The dust from clay litters can cause respiratory problems. Clumping litters can cause nasal blockages if the ferret likes to burrow in the litter and many of them do. In my experience, the pine pellets control odor better than the newspaper pellets.
You need to check this baby for fleas and earmites. If he/she has fleas please do not use any of the stuff sold over the counter for fleas. Get Advantage, Frontline, or Revolution. Revolution will also take care of an earmite problem and is a heartworm preventative too. You will be able to tell if he/she has earmites by cleaning the ears. If there is black, coffee ground looking stuff in them, there is a mite infestation. You can use Rev like I said or Tresaderm or Ivermectin for earmites.
Good luck finding her owner. If he/she wasn't intentionally set loose, someone is probably frantically looking for him/her.
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I've learned that I don't suffer from insanity, I enjoy it.