Oh, hey, didn't the Yeerks capture that one Ongachic ship in the Hork-Bajir Chronicles? Possibly more, thereafter? And doesn't Ax describe Klingons as looking like "Ongachic females" in #8? Klingons are kinda wrinkled in the face-region, and if they're standing next to a Hork-bajir they could be called 'little,' so if that's what Ongachics look like then they could be the 'little wrinkled ones' mentioned.
They'd be rare enough to explain why they're never mentioned again in the series (being a nomadic race, the Yeerks would never have been able to conquer them in large numbers), but it's also possible that the Animorphs would have seen one or two of them prior to #5, explaining why Cassie speaks as if she's seen them before.
Also, keep in mind that even if other aliens aren't mentioned, that doesn't mean they aren't there. Animorphs is a fast-paced, action-oriented series, and the authors generally try to minimize unnecessary description. They mention the aliens that they know the names of, or that they interact with (Hork-bajir being the main combat troops, and Taxxons who can't help but to get involved with battles as soon as they smell blood), because to describe every alien in the background would take too long, and there isn't much point. Hey, the poor kids are already juggling superhero life with schoolwork, you want them to start compiling encyclopedias of alien life forms, too? Consider how rarely Gedds are mentioned, even though we know they're there. They just aren't as important to the story, and for the most part the Animorphs didn't know (or care) what they were.