I know that the original host species of the parasitic Yeerks was the Gedd species.
Who was the first non-Gedd individual to be taken by a Yeerk? How did it happen? Was it before or after Seerow's Kindness?
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While the first species the Yeerks successfully conquered in large numbers after the Gedds was the Hork-Bajir1, they made abortive attempts with two other species before finding the bladed aliens' homeworld.
The Hawjabrans were unable to be infested in a meaningful way due to having nervous nodes in multiple locations through their bodies instead of a single brain. The Ongachic were successfully infested, but the race had long since abandoned their home planet and become wandering nomads, leaving the Yeerks with highly limited numbers of them to use.
It's unclear which of these two species were attempted first, but as the Hawjabran could not be infested successfully, the first successful non-Gedd Controller would have been an Ongachic.
But then it happened. Palp to palp, the message came to me. Esplin 9466 to the infestation pier! There was a new species to try. After failures with the Hawjabrans and only the few Ongachics, our wandering assemblage of spacecraft had found a new planet. With new creatures.
Hork-Bajir Chronicles, pg 32
1As mentioned by @IG_42, book number 8 (The Alien) mentions a race called the Nahara as being conquered before the Hork-Bajir. However, these are never referenced again during the series - including in The Hork-Bajir Chronicles, which take place during the pre-Hork-Bajir period - so this is likely just a continuity error.
It's unclear, but it seems that two "test" Hork-Bajirs were the first non-Gedds to be enslaved.
As far as I can, immediately after Seerow's Kindness and the Yeerks betrayal, the Yeerks invaded the Hork-Bajir Homeworld.
From the Hork-Bajir page:
The Hork-Bajir homeworld was the first world the Yeerks reached out to for conquest.
Then, from the wiki page on Esplin 9466:
Soon after arriving at the Hork-Bajir Homeworld, the Yeerks capture two specimens to examine for their fitness of infestation. Esplin is called to the pier, and becomes the first Yeerk to inhabit a Hork-Bajir as a permanent host. He utters the words "Yes, these creatures will be our weapons," and seals the fate of the Hork-Bajir.