In the last post, I talked about how Lent began to be observed in Christianity from the time of the Apostles to the end of the first millennium. In that millennium, Lenten fasting and abstinence changed but little. So did Christianity itself, relatively speaking. There was no split between Eastern Orthodoxy and Roman Catholicism yet, and neither did any Protestant denominations break off and diversify from Catholicism, though the Oriental Orthodox churches broke off. However, most Christian denominations today find their origins in the second millennium whether they claim to be the ones that kept the original faith passed down from Christ or not. Where I'm going with this is that since it's a gargantuan task to track how fasting and abstinence has changed in all Christian denominations, I will just focus on Catholicism.
By the second millennium, fish was on the menu during Lent for the West at least, according to Alban Butler's book, The Moveable Feasts, Fasts, and Other Annual Observances of the Catholic Church. But what counted as fish? Apparently, Catholics of the late Middle Ages counted more than just those animals with scales and fins that live in the water. Even Eastern Christians have acknowledged shellfish as permissible to eat during Lent despite forbidding true fish with bones. Therefore, by "fish" Catholics really meant "seafood". So, what counted as seafood? With no evolutionary biology to guide them, philosophers of the time looked more at the behavior or habitat of the animal rather than its physical characteristics. Therefore, anything that mostly or completely lives in the water could count as fish. Waterfowl such as ducks, however, did not, since poultry such as ducks and geese were served at feasts just like a ham, and abstaining from festive foods was the whole point of Lenten abstinence. That being said, even some mammals were counted as fish, and I'm not talking about whales. Instead, I'm talking about beavers. Apparently, even though beavers are clearly mammals and have fur, European philosophers considered beavers fish-like enough because they have furless, "scaly" tails and spend a lot of time in water. By the 12th century, beaver tail was on the menu during Lent in Wales and probably other areas in Western Europe as well. This precedent would make for some unusual inclusions of other animals as "fish" later on.
Fasting rules were loosened during the Middle Ages as well. Formerly, no food or drink was allowed until sunset, when just one full vegan meal was eaten. Later, the one meal was turned back to 3 p.m., and in the 14th century, the meal could be eaten as early as noon. Then, in the 15th century, a "collation" could be had in addition to the meal. A collation is basically a snack and drink, and it could be consumed in the evening. Presumably, adding the collation meant also that you could drink something whenever needed as well. If a collation was added for keeping strength up, I would imagine that clerics would have noticed how dehydrated people were getting and would have allowed more drinking (or at least that's what I'm guessing - I never found anything about when drinks were allowed at any time of day).
When the Western European countries began expanding overseas to become colonial empires, this led to new modifications of Lenten abstinence. Pope Alexander VI allowed Spain and its colonies to eat dairy products during Lent, for a reason unknown to me. When French people colonized Quebec, they were soon dependent on beavers not only for trade but apparently for food as well. Beaver tail must have still been a grey area for them, because in the 17th century they needed their bishop to make sure it was okay for people in Quebec to eat beaver tail during Lent (and it was). Another furred mammal found itself classified as a fish on the other side of the Americas: capybara. This may have been a special dispensation for the indigenous population that hunted capybara a lot in Venezuela, or just another excuse based on the fact that capybaras are semi-aquatic. Likewise, there's a rumor that Catholics of the Detroit area have been permitted to eat muskrat, another semi-aquatic rodent, during Lent since the early 19th century. Other areas eat (or used to eat) cold-blooded reptiles in Lent as fish, such as alligators in Louisiana today or turtles at a restaurant in Melrose, MN until less than a decade ago.
The next global change to Lenten abstinence, however, came in 1741 when Pope Benedict XIV allowed meat and dairy products to be eaten on some of the days of Lent, rather than abstaining from those for all 40 days. By the end of that decade, the 1740s, fasting and abstinence rules were few enough that some Catholics could be ridiculed for having sumptuous meals that they still called fasting. For example, we have Peter Kalm's description from 1749 of the "fasting" that Catholics in Quebec were doing:
Friday and Saturday, the 'lean' days, they eat no meat according to the Roman Catholic rites; but they well know how to guard against hunger. On those days they boil all sorts of vegetables like peas, beans and cabbage, and fruit, fish, eggs, and milk are prepared in various ways. They cut cucumbers into slices and eat them with cream, which is a very good dish. Sometimes they put whole cucumbers on the table and everybody that likes them takes one, peels and slices it, and dips the slices into salt, eating them like radishes.... In brief, they live here just as well on Fridays and Saturdays, and I whom am not a particular lover of meats would willingly have had all the days so-called lean days....
Beaver meat is eaten not only by Indians but ... especially by the French, on their fasting days; for his Holiness the Pope has, like many of the old zoologists, classified the beaver among the fishes, since he spends most of his time in water .... In connection with the eating of beaver the fasting of Catholics appeared to me a bit strange: Those who first inaugurated the fast days did it undoubtedly with good and holy intentions to keep the people from eating too much meat, which is injurious to health, fattens the body too much, and makes it inadaptable for many things. But ... if they could afford it they lived everywhere sumptuously and fed their body just as on the other days of the week; for they then had more courses prepared of eggs, of all kinds of fish, prepared with oils and fats; all kinds of milk dishes, and many especially sweet-tasting fruits with a quantity of wine. So that for the most part wherever you ate on a fast day the table was better provided with varieties of food than on any other days, and still they called it fasting. (Peter Kalm, Peter Kalm's Travels in North America: The English Version of 1770)
Keep in mind, there was still only one meal every day allowed at this point, though a collation could be eaten too. However, in the 19th century, a second collation was introduced which could be taken in the morning. This gave rise to the clarification that the two collations are supposed to add up to less than a full meal. Even so, this meant you could eat three times a day and still call it fasting if two of your meals were snacks. Dairy products and eggs were still prohibited during Lent in some dioceses as late as 1913, but now it's only the Eastern Catholic churches that have that restriction, just like the Orthodox churches. In 1917, an edition of the Code of Canon Law was published which clarified for the whole Catholic Church that the one meal and two collations could be had at any time of day, and those under 21 are excused from fasting.
After the Second Vatican Council in the early '60s, even more changes happened. In 1966, 40 days of fasting and abstinence were reduced to just 2 days of fasting (Ash Wednesday and Good Friday) and 8 days of abstinence (Ash Wednesday and all Fridays of Lent). In 1983, two minor changes were made: one for the whole Catholic Church and one for just the Catholic Church in the United States. The first, a part of the 1983 Code of Canon Law, declared that solemnities such as the Feast of St. Joseph (March 19) and the Feast of the Annunciation (March 25) override Lenten fasting and abstinence on those days (except if March 25 is during Holy Week, in which case the Feast of the Annunciation is moved). St. Patrick's Day is also considered a solemnity for Irish Catholics and many Irish Americans. In the United States, the United States Council of Catholic Bishops defined age limits for fasting and abstinence: 18 to 59 for fasting and 14+ for abstinence. They also said that Fridays outside of Lent, traditionally days of abstinence too, could involve other acts of penance rather than abstinence. Unfortunately, most American Catholics forgot the part about another form of penance and thought that the USCCB simply dropped the requirement of abstaining from meat on Fridays outside of Lent, full stop. You can read more about what the USCCB actually said here. Also, the USCCB warns against making Friday abstinence too tame, saying that "indulging in the lavish buffet at your favorite seafood place sort of misses the point." I wonder what Peter Kalm would have thought of Lenten fasting and abstinence in the United States today.
In short, the second millennium started with 40 days of eating just one meal in the evening with no meat, eggs, or dairy (but maybe fish and certainly shellfish were okay). The second millennium ended with just two days of "fasting" meaning one full meal and two partial meals, and no "meat" on those two days and Fridays (but eggs, dairy products, and anything aquatic or semi-aquatic except waterfowl are okay). Lent became quite tame by comparison in the Roman Catholic Church, though not as much in any of the Eastern Catholic churches in full communion with the pope.