You are looking at a very nice RALPH LAUREN Blue label blue/white/gray plaid long sleeve button up shirt size Small (see measurements in photo and below) or youth 18-20 per the tag, that is regular fit and in very good condition. The measurements compare to an adult size small easily, which is why it can fit as an adult size Small in addition to what the tag states.
The pony logo is a colorful one, where the polo player actually is wearing a uniform versus is one solid color.
Please see all photos for details. Due to various lighting conditions during photography, color may vary a bit.
Measurements are:
Size S or youth 18-20
27" length from top of shoulder to bottom of shirt
19.5" - 20" width from armpit to armpit
24" sleeve length
2" cuff length