Strictly speaking, Ratatoskr could not be considered one of the Earth Ancient Gods. She belonged to a special kind of existence. Like the Giant Eagle, Black Dragon, boar, and goat of Nine Realms legend, she was one of the powerful creatures born upon Yggdrasil.
Back when Yggdrasil was rooted in ancient Earth, there was nothing wrong with her calling herself an ancient god.
Angerboda had been laid as an egg by her parents on the ground beside the roots. In essence, she was one of the Old Ones, but it would not have been wrong for her to call herself an Earth Ancient God either.
At the time, Oshtur had still been a little blue bird, flying around for fun and occasionally resting on Yggdrasil. She was an Earth Ancient God too.
But when Ratatoskr told the Flower Fairy Queen that she was an ancient god of Earth, she was just gilding her own reputation. Her seniority was not that high; she should have been a generation below even the Leopard God and Tiger God.
Millions of years after the First God War came to an end, with Oshtur and Gaea's side victorious while Chthon and Set fled, Yggdrasil and Angerboda began their journey toward the Inner Planes.
During that process, Yggdrasil expanded its body and refined the various concepts its roots absorbed from mountains of corpses and seas of blood, thereby creating the Nine Realms, including Hevenheim.
The squirrel Ratatoskr had probably been born around that time, as the representative of the two neutral concepts of "mischief" and "messenger."
If one insisted on counting it, she could probably be considered a god, because she was genuinely powerful—perhaps even stronger than the Leopard God.
But her hometown was Yggdrasil itself. At that time, she had not even touched a speck