Prepare for a wall of textI like the juxtaposition it creates. Pegasi, a race that normally embodies the spirit of freedom, and lives to the motto of 'the skies the limit' being bound, or grounded, when they find the one. The knot signifies the bond between the couple, and tells of an unspoken promise that they will settle down with each other, make a nest, raise a family and then grow old together. I have this head cannon that Pegasi only have one mate, and they stay loyal to that pony until the end.I'd figure it started sometime around the rule of Hurricane, after the unification of the races. The Pegasi had originally been a very traditional, nomadic, almost viking like race; whose community thrived under a strong militaristic rule. The stallions would go off to fight for glory, claim new lands, whilst the mares would look after the yearlings.Once they had served their time, their family would move to a new colony and help establish it.After the events of the first Hearths Warming, the races began to mingle much more, and wars and conquest were no longer an agenda. Instead ponies began to settle down, some finding love with another race. In these ceremonies, a Unicorn or Earth pony would tie themselves to the Pegasus, as a way to signify them giving up the open skies, and to implicitly say "". This tradition then became popular with the pegasi, as a way of signifying their bond with another pony, by giving up some of their most cherished asset (Freedom). As the years went on, the tradition became woven in Pegasi culture, and is still carried on in modern ceremonies.Of course, this is all just my head cannon