Attack on titan Opening 3 (Blu-ray 1080p)

I do not own any rights to the music or video used

[elbladel]

So many years later... we got all the answers.

[OysterTran]

Still my favorite opening in this series

[Weldingization]

For the folks wondering about the t-rexes and whales and stuff (yes, there's a lot of you) here's an explanation :
They're not literal animals, and are unrelated to the beast titan directly: it's about themes & artistic intent.

The theme of the opening is the creed of the scouts: 'shinzou wo sasageyo', "dedicate your heart". As we see the characters stand in salute at 0:55, we see a bright spark over their hearts: they've fully dedicated their hearts to their purpose. However, it's revealed at 1:13, when the armored titan appears, that it's not just the main characters that have dedicated their hearts: we see a bright spark at the armored titan's heart, too. Despite being an enemy, it has also dedicated it's heart fully to some purpose (if you know the entire story, you'll understand why).

After that, we see a bunch of flashes of different prey animals and predator animals, from a fly being eaten by a chameleon to an orca eating a smaller fish. We even see insects and dinosaurs hunting one another. At 1:19, we even see microscopic organisms fight for their survival!
For each of these animals, we see that same spark over their hearts: both prey and predator are fully dedicated to their purpose, and their survival.
Like Mikasa said: the world is cruel. For a predator to survive, a prey must die.

After this, we see the beast titan, a bright spark over their heart, running towards the wall, accompanied by a plethora of prey and predator animals.
They're not really marching with him: they are there to artistically illustrate the themes reinforced by the imagery so far: that the beast titan has the same purpose as creatures big and small have for all of history: to ensure it (and its species') survival by dedicating their hearts fully to that purpose.

TL;DR : The animals represent the ancient will to survive, which is carried to this day by all the show's characters.

(Bonus note: It's interesting that the only creatures without a bright spark over their hearts are the non-intelligent 'pure' titans! They do not hunt for their survival: in fact they care very little for anything at all. All they do is consume for no reason whatsoever. Plus, if you know all of the story ((SPOILER)) you'll know they are people turned into titans against their will.)