What are the Deviants doing on Earth?

The Deviants were sent by Arishem to certain Celestial host planets prior to 5000 B.C.. They are seen killing dinosaurs, which will support their intended purpose of clearing out predators so that intelligent life will arise.

ARISHEM: I created the Deviants, Sersi, for the same purpose I created you. Every Celestial host planet has its own predators. I first sent the Deviants to exterminate them so intelligent life can grow, but there was a flaw in the design. They evolved, became predators themselves. And I lost control of them.

They had begun to be a menace to intelligent life by 5000 B.C. (Babylon) and so the Eternals were sent to eliminate them. A tipping point was reached where Deviants were consuming humans in such amounts that the coming of Tiamut was being significantly retarded.

ARISHEM: In order to grow, Tiamut needs vast amounts of energy from intelligent life. The Deviants prevented this by consuming humans until the Eternals eliminated them.

It seems most likely that Earth was one of the planets that seeded with Deviants by Arishem. It may be that with the arising of sophisticated Babylon in 5000 B.C., humanity passed the developmental threshold at which their intelligence could contribute to the coming of Tiamut. At that point Arishem could no longer tolerate the ravaging raids of the Deviants on humanity and the Celestials were sent to eradicate the Deviants.

However I note that the Marvel Cinematic Universe wikia entry hedges its bets here, stating that Deviants 'spread across the galaxy'.

Having lost control of the Deviants, who were now running rampant across the galaxy, Arishem created a new race to combat them, synthetic rather than organic, and named them Eternals. It became the role of the Eternals to exterminate the Deviants and protect seeded worlds, and Eternals soon hunted Deviants across the galaxy. Multiple Deviants came to the planet Earth seeded with the Celestial Tiamut, prior to 5000 B.C, and became feared by the local human population.

So from the point of view of the (usually reliable) MCU wikia, Deviants 'arrived' from some third party planet, which suggests that Deviants are capable of space travel and were not placed on Earth by Arishem.

However this seems unnecessarily speculative to me. From in-movie dialogue alone, I am satisfied that the Deviants were placed on Earth by Arishem.

The movie and the source material differ widely on the Deviants.

Source Material

In the source material, Celestials select the dominant race of a given planet and alter their genetic structure for reasons that have not yet been revealed. The Celestials use their advanced technology to alter the race into three distinct races:

a) The mainline race that shows normal attributes until evolution and an outside catalyst creates mutation, granting individuals of the mainline race a wide array of varying abilities.

b) A superior race dubbed Eternals who are typically empowered with cosmic energy, and

c) a genetically unstable race dubbed Deviants. The Celestials have practiced these tests on various races such as Humans, Kree and Skrulls.

When the First Host got to the planet Earth, the Celestial Gammenon the Gatherer collected a few specimens, which Ziran the Tester mutated to create one hundred grotesque Deviants (Homo descendus). After Ziran was done with its brood, Nezarr the Calculator performed his own tests and created 100 superior Eternals (Homo immortalis). Finally, Oneg the Prober created a latent gene for the expansion of human potential.

After these creations were released back on Earth, the Eternals secluded themselves, while the Deviants established an empire on the continent of Lemuria. The Deviants multiplied quickly, while the Eternals mostly hid for a while, tried to fight the Deviants and eventually called the Celestials for help.

MCU

In the MCU, The Deviants were a race of genetically engineered organic beings, created by the Celestial Arishem the Judge in order to assist in the process of Celestial seeding. Arishem created the Deviant race with an intended role; to wipe out the apex predators of seeded worlds and thus allow sentient life to evolve. For a time, the Deviants succeeded in this role, killing of the predators of many worlds. The problem was, the Deviants eventually became the apex predators and began killing all the humans, and as a result the Celestial wasn't getting fed. It is never specified where they were created, or how they arrived at their destination worlds. It is known, however, that they arrived sometime before the Eternals and the Celestials had realized they were killing the native Human population at an accelerated rate.

In 5000 BC, ten superpowered Eternals—Ajak, Sersi, Ikaris, Kingo, Sprite, Phastos, Makkari, Druig, Gilgamesh, and Thena—are sent by the Celestial Arishem to Earth on their starship, the Domo, to exterminate the invasive Deviants. In 1521, the last of the Deviants on Earth were hunted down and killed by the Eternals. However, in Alaska, a group of them that included the leader, Kro, had become frozen in ice, and remained so for centuries.

In 2023, due to the Blip, the ice defrosted, and Kro and his pack were set free. The Eternals learned that they were never supposed to kill off the Deviants, but rather prepare Earth for the emergence of a Celestial.

As you can see, the movie and the source comics are pretty far apart on what The Eternals were even supposed to be doing on Earth. They also differ widely in that in the source material, the Deviants were created on Earth by Ziran the Tester, whereas the movie never specifies where the Deviants were created, just that they were created by Arishem the Judge and later sent to Earth.