Intertrigo. Very common in babies , seen in between folds- neck , armpit areas, groin , behind knees and ears. Due to moisture and irritation . May be mild to very severe (very raw excoriation). Need to keep areas dry. Anti fungal creams usually help.
Common causes... ...Include atopic dermatitis (a type of eczema), seborrheic dermatitis, or a yeast infection. There are other less-probable possibilities as well. Without more details about the rash and symptoms i can't be more specific. You can try 1/2% Hydrocortisone cream, but that will not help if it's a yeast infection. See your pediatrician or a dermatologist for a precise diagnosis.
Rash on neck. A rash in the folds of the neck of an infant is often caused by moisture and skin touching skin. It is one of several possibilities intertrigo, atopic dermatitis or seborrhea. I would take care in drying out the area well and put corn starch powder on it to keep it dry. I f it is not improving you may want to see your pediatrician or a dermatologist to see if you need a mild steroid cream.