This white, round pill imprinted with 503 1mg is a Intuniv 1 MG 24 HR Extended Release Tablet, per the NIH.
Intuniv is a human prescription drug. The active pharmaceutical ingredient is guanfacine hydrochloride and is classified as, "Adrenergic alpha2-Agonists [MoA], Central alpha-2 Adrenergic Agonist [EPC]."
This pill is made by Shire US Manufacturing Inc.
There are 9 other white pills that with a similar imprint: "503 1mg". They may be a different dosage of the same drug or a different drug entirely.