According to The Oxford Dictionary of the Christian Church, "during the early centuries the observance of the [Lenten] fast was very strict."
The reference work says that in the early Church the Lenten fast allowed one meal per day, taken towards evening, and that "flesh-meat and fish, and in most places also eggs and lacticinia, were absolutely forbidden," but that the practice "began to be considerably relaxed" in the west from the 9th century.
The 12th century Decretum Gratiani, a compendium of ecclesiastical law, includes the text of a letter which was believed by Gratian to be from St. Gregory the Great to St. Augustine of Canterbury. This letter says that during Lent "we abstain from flesh meat and from all things that come from flesh, as milk, cheese, eggs." A critical edition of the Decretum calls the source of the quote Pseudo-Gregory, and according to Dr. Mark DelCogliano, an associate professor of theology at the University of St. Thomas, the quoted text "first appears in Gratian."
Writing in the late 13th century, St. Thomas Aquinas said that it was "common custom" that those fasting abstained from meat, eggs, and dairy products, but were allowed fish.
It is said that the practice of calling "Fat Tuesday" the day preceding Ash Wednesday derived from a period when the use of animal products was barred during Lent. "Fat" Tuesday was thus the last day to use up the meat, cheese, and animal fat stored in the home.
And while not precisely vegan, the traditional Byzantine fast barred the use of meat, dairy, eggs, and fish. Animal-based foods that were permitted included honey and invertebrates.
In the Byzantine rite, Lent is preceded by a pre-Lenten period known as Fore-Lent. The last two Sundays of this preparatory period are known as Meatfare Sunday and Cheesefare Sunday.
Under traditional fasting rules, Meatfare Sunday was the last day before Easter to consume meat, and Cheesefare was the last day to use dairy products. The Lenten fast then began on the Monday after Cheesefare.