Ok, so the arn are introduced in the HBC. They were the ones who created the hork-bajir, and were maters of creating life. Their biological technology was super advanced, able to create an entire species of hork-bajir and other various monsters to keep hork-bajir from their valleys. It was mentioned that they were able to control how fast their creations matured too, the growth rate. From at the beginning of creating it, they could modify the cells or whatever to make it grow and reach adult faster and what not (think it was mentioned in HBC or #34 the prophecy).
When the yeerks invaded the Hork-bajir homeworld, they knew of the arn, and there were countless arn valleys/colonies spread all over the planet as Dak said. The yeerks definitely eventually discovered the arn, and then arn eventually began to modify themselves genetically to make them useless to the yeerks as hosts.
And so, here is my question - Why didn't the yeerks simply use the arns technology to create more hork-bajir hosts and an entirely new species altogether for them to use as hosts?????!??!!? Surely they would have been able to capture some arn as successful hosts and use their arn memories and knowledge before the arn managed to alter themselves, I mean, it wasn't an overnight process (the arns modifying their ears or wateva) was it? And once the yeerks learnt that they couldn't use the arn as hosts, they used the arn as shooting target practice......surel y the yeerks could have made some deal with some arns to get them to create more host bodies or species for them to infest, or at least force them to do it aggressively or force them to teach their skill and craft to the yeerk scientists or yeerks who are smart. And even if this didn't work, the yeerks seemed already well advanced and educated by the time they invaded the hork-bajir planet in HBC. Couldn't their yeerk scientists or whatever salvage the arns computers/data/knowledge and begin trying to create more hork-bajirs and other life forms to use as hosts for themselves??
And in what is seen in #34 the prophecy, there are still arn laboratories left undamaged throughout the planet. Surely in the what, 60 years of staying on the hork-bajir planet, the yeerks could/should have been able to look into and explore the arns technology and knowledge. Based on the HBC, the hork-bajir planet was like first planet the yeerks successfully and totally conquered, even before the taxxon planet.
By the end of the HBC, we know that the yeerks are fully aware of the arn valleys and civilization.
In the years after, they should have been able to simply locate all the arn labs with their ships censors as they advanced.
Hard to believe that in all this time - with all the silly experiments that we see the yeerks on earth did, all the yeerks advanced technology and their scientific/technical knowledge, and all the smart yeerk scientists we see experimenting, creating and doing things throughout the main animorph series - that none of the yeerks thought or bothered to look into and use the arns life-form creating biotechnological/cloning/genetical engineering knowledge and capabilities. After all, as Ax said, the yeerks are scavangers, they steal technology from other species and learn it and modify it for their own benefits. With the arns knowledge and capabilities, they wouldn't have to bother go searching for and conquering other suitable host species at all - they could have simply made bodies for themselves and give whatever the heck they like the body to have! such as a body with three andalite tails, or a hork-bajir that could spit acid, or a multi-tentacled tall muscled flight capable uber creature or whatever, the possibilities are endless!. I know its wouldn't be easy for the yeerks to do it straight away, and creating healthy specimens etc. would take alot of work to ensure and reach the point where it is free from abnormalities and defects. But we already see that its entirely possible cause of the hork-bajir and other creatures which the arn had created. And the Yeerks literally owned the hork-bajir home world, with the arn and their stuff at their disposal for all that time.
And We don't even know what became of the hork-bajir homeworld by the end of the series. nothing was mentioned in book 54. for all we know, it could still be under yeerk control.