Actually there have been well over one hundred different Allen branches who have unique coats of arms. Allens usually come from Scotland, with ties to England, France, Germany and Wales and even Ireland. In each of these areas a different and almost wholly unrelated patriarch had his own coat of arms. The laws of heraldry in these countries, and every country in the world that has had a history of Royalty granting arms, state that a "coat of arms" is never given to a family. It doesn't work that way. They are granted to an individual man by a personage of Royalty who has the authority to grant such honours. Because of this, different men with the name Allen have each had their own heraldry, which is only passed on to the eldest male heir, who passes it to his eldest male heir. The female line is a different story, though it is usually recorded that their father bore those arms, unless the female is so high a peer as to be a Princess, Duchess or Countess, she almost never inherits the arms of her father, and almost never uses arms herself in any way.
As many men with the surname of Allen that have heraldry, many many more do not. Because one man had arms does not mean that all Allens have. It is nice to know the heraldry of an Allen who bore arms that we might be related to, whether closely or distantly though.
In that case a person should rightly trace their line to or near one of the Allen men who had been granted a coat of arms at some time.
Irish and Scottish geneologies and the Lord Lyon King of Arms in Scotland are the best ways to find this out. Following heraldry closely for many years, I have a long list of Allen men who actually have been granted arms. All are from before the 1600s, and all are from rolls of Arms or Royal registries in France, Scotland and two from Ireland (moved from Scotland).
But to connect a modern Allen family honestly to these men, descent should be able to be traced back to at least the late 1500s, which many Allens I've met are having a hard time doing. Many Allen families I know of lose the line somewhere around either the Revolution or upon coming to the United States. Only the surviving families I know of in Scotland and one in England have any long record, and one of those families can trace father to son all the way back to Adam.
For a long time in England and Scotland some things that the Allen men had in common on most of their individual shields were the colors Red and Black and sometimes Silver and Gold. Of all the various ones I've seen through the years, the primary color seems to be Red (Gules in heraldic terms). Also, some of the later grants seem to be to a large group of Allen men living all over the Isle of Great Britain, but all remotely related. These were granted from 1497 to 1608, and most have in them somewhere a type of hunting dog called a Talbot. This was the dog used very much for hunting and research has shown that many Allen men were the Royal huntsmen for several Scottish clan chiefs. This means that they were in charge of the stables, all elements of hunting, which included hunting for entertainment and also regular provision of meats for the chiefs' tables. To signify this, the Talbot was usually an important part of the coat of arms, and is a crest (the device on top of the helmet) for many Allen men of this period. Over 20 Allens from this period who were entitled either as barons or knights were given arms with this motiff, usually a predominantly Red field with Black (Sable) talbot or talbots somewhere on the shield.
On the other hand, several men with the surname of Allen, who had either come over with William the Conquerer, in his army as knights, or descended from such knights, used as primary colors Blue (Azure in heraldic terms) and Silver (Argent) and have the Norman symbol of either a Star (ettoile) or Spur (mullet) or Shell (escallop) or sometimes many or combinations of these symbols in the field of the shield.
The very few Allens who bear any relation to Walter FitzAlan, or Robert FitzAlan, the first Stewards of Scotland and lords of Arundale, bear combinations of the original arms bourne by those famous men. The symbols are usually a Red shield with a Gold ("or" in heraldry) Lion Rampant, some have stripes (barry) of Red and Gold. Some have checkered (chequey) sheilds of Red and Gold, which look like a chessboard. The FitzAlans came from Normandy with William the Conqueror, and before, in Normandy, were within a land ruled by the Belgian rulers who bore Blue and Gold in a checkerboard pattern.
The Stewarts of Scotland have in their shields usually the same blue and gold checkerboard patten, alluding both to their jobs as Royal clerks (counting boards used for counting things such as money) and also the ancestry from Northern France before the 1000s A.D.
One line of Allens moved from the lands of Arundale in Southern England to Western Scotland, and became a sept of the clan MacDonald (Donnel, Donald, Domnhuil are all variants of the same clan name). Under Alan MacDonald, brother of the ruling chief Ranald, their patriarch bore a Silver shield with a Red Scottish warship (the symbol of the MacDonald chieftain) whose masts, instead of bearing sails, bore a smaller shield which was Red. On this smaller shield were the more ancient symbols of the FitzAlans, a Golden Lion Rampant across whose middle was a checkerboard pattern of Blue and Gold.
This family went on the First Crusade of Christain knights to Jerusalem. After their return to Scotland, descendants of this family usually had various versions of the ship, the lion and the checkerboard, and most bore as a crest on their helms a Golden Cross Potency, which is a Cross whose ends terminate in "T" shapes.
Two Allen men from England have versions of an Anchor or Lion on their shields in Chief, which means in a different colored area at the top of the shield. One of these men has a Black Anchor between two Black Talbots, each within Silver circles on a Red Chief, while the field of the shield is a Gyronny pattern of Gold and Red.
So, You can see, there are many different shields, all depending upon which man each Allen family is related to. Some Allen families aren't related to any of the men who have ever been granted arms.