Generation 1 canon was played out in two different animated series, a theatrical movie, comic books, children's books, and backcard stories (toys). Different sources would sometimes contradict each other or sometimes simply not acknowledge other works.
The two G1 animated series had little to do with each other.
My Little Pony 'n Friends is set in Dream Valley in the fantastical world of Ponyland. The primary group of ponies lives in Paradise Estate, and they often go on adventures where they meet magical creatures, fight evil villains, and stave off the end of the world on a surprisingly regular basis. In-between these adventures, the ponies can be found playing games, baking sweets, and telling stories, among many other activities such as the Pony Olympics.
Outside of the main pony group, a few more ponies can be found scattered around. The Big Brother Ponies are on a race around the world. The Princess Ponies live in a faraway castle where they guard their magic wands. And Scoops works at an ice cream parlor that is seemingly separate from the rest of Paradise Estate.
Other equestrian species also live in Ponyland, the two most notable of which are Flutter Ponies and Sea Ponies. The Flutter Ponies of Flutter Valley are very similar to the little ponies, except for their fairy-like wings and lither builds. Sea Ponies, creatures with the heads of ponies but bodies more akin to seahorses, live in the rivers. There are a few other seemingly closely-related species as well. In the town of Grey Vale, previously known as Bright Valley, live a group of equines that are bigger than the little ponies, have noticeably defined hooves, and lack any sort of markings or patterns on their sides. Other similar species include a more realistically-proportioned horse ridden by a knight, a donkey, a bipedal zebra, a camel, and a magical goat.
My Little Pony 'n Friends, as well as much of early G1, is notable for being the only instance of humans living in the same world as the little ponies. While the ponies do need to cross over a magical rainbow bridge to find Megan and her siblings specifically, other humans can be found throughout the ponies' world as well.
While very clearly set after My Little Pony: The Movie from 1986, My Little Pony 'n Friend's relationship to the earlier TV specials, Rescue at Midnight Castle/My Little Pony in Dreamland/Firefly's Adventure and Escape from Katrina/Catrina, is less clear. People, items, locations and certain creatures are consistent throughout all instances, such as Megan, Dream Castle, the Rainbow of Light and Spike the dragon, but the ponies are not. None of the ponies from the original special are ever seen or heard from again, and the few ponies in Escape from Katrina that do appear in My Little Pony 'n Friends both look and sound slightly different, suggesting some sort of separation between the 1986 movie/MLP 'n Friends continuity, and that of the first two specials.
While My Little Pony 'n Friends is set in a more clearly medieval fantasy-inspired world, the Ponyland of My Little Pony Tales is very much like our own world, if our own world was still in the 1980s/90s. The ponies now act much more like humans than animals, going to school, having jobs, often walking on two legs, and using modern technology.
While at first glance it may seem that My Little Pony Tales is as far from the world of My Little Pony 'n Friends as you can get, there are some occurrences that hint at a deeper connection. In as early as the first episode, the pony Patch tells a story about knights and castles and dragons, and one of the knights later appears as a ghost, suggesting the story to be true. In a later episode, magical ponies, the Glow 'n Show Ponies, disguise themselves as a UFO and save one of the characters using their powers. These fantasy elements, though few and far between, mean that there could be something more to the world of My Little Pony Tales than there appears to be on the surface.
Plants and Animals
Some of the books published in the UK were canon with the UK comics. Most other books, i.e. books published in the US were not canon with anything.