Man this series makes no fucking sense
Well of course it makes no sense, you’re watching it out of order.
Obviously, the first one is Rise of the Guardians. It shows the Guardians group in its early days, as well as the process of becoming a Guardian, and that Guardianhood affects animals as well as humans.
They did a spinoff, The Guardian, which was supposed to be a gritty, adult, humanocentric version. It is widely considered a flop, in large part because it cut out the ascended guardian actually joining the group, and played the Guardians off as a myth.
After a skip forward in time, there’s Guardians of the Galaxy, which shows the Guardians at their peak of power, but also lacking the understanding of the supernatural forces involved in becoming one. You can see that animals are still subject to forces like this, and now even plants.
Finally, after the slow collapse of the decadent Guardian group-complex, the Legend of the Guardians tells of its revival on a far-flung planet dominated by owls and other birds and rife with the Guardian force, causing widespread anthropomorphisation. The ancient Guardian group is now legendary and widely considered a myth, but the new group looks to be off to a promising start.