Father Guy Selvester is an international expert on ecclesiastical heraldry. He recently wrote an outstanding article on his blog Shouts in the Piazza on the issue of priests having their own coat of arms. Rather than being a sign of pretentiousness or vanity, ecclesiastical heraldry is about tradition and honor.
During Mediaeval times, the clergy and nobility (first and second estates, respectively), used heraldry to tell a story about the person, their background, their origins, etc. The heraldic shield came from the actual shield knights used to protect themselves while wearing armor and engaged in battle. Ecclesiastical heraldry is as old as that of nobility and monarchy. Steeped in tradition, today's coat of arms, especially in ecclesiastical usage, connects the past with the present. Family symbols are used when possible and creatures and things which represent places, ideas, values, and deeds.
Most people know that Bishops, Archbishops, Cardinals and Popes have their own unique coat of arms but what most people do not know is that priests (and deacons) as members of the clergy are equally entitled to their own as well. The only difference is that everyone who enjoys the fullness of Holy Orders (episcopacy) has a diocese or titular see under their jurisdiction, hence, half of their coat of arms is that of the particular place with the other half being their own or familial shield. The personal side is usually designed for the cleric when he is ordained or consecrated bishop.
Deacons have a black hat (galero) with no cords or tassles whereas a Priest has the same black galero but with two cords (one on each side) with one tassle each. Vicars Forain (Deans) and Episcopal Vicars have a black galero and two cords but with two tassles on each side. Monsignors have the following: Chaplains of His Holiness, black galero with six tassles on each side; Prelates of Honor, purple galero with six tassles on each side; Protonotary Apostolic has purple galero with six red tassles on each side. Bishops have green galeros with six tassles on each side; Archbishops have green galeros with ten tassles on each side. Cardinals have a red galero with fifteen tassles on each side. Popes have the Papal Tiara.