Nope, it's not an Andalite/Hork-Bajir mix. This is what I think an Ssstram would look like.
Four ears, four eyes, and very colourful. It seems like this version of the Ssstram would be a Yeerk's ideal host. I mean, Esplin 9466 and Aftran went on about how great sight was. The Ssstram homeworld would have been a beautiful planet by the time the Yeerks arrived.
They're capable of flight, thanks to the hair/feather-like things around the ears and arms, and have a prehensile tail. Their arms are much longer than that of Andalites, humans, or Hork-Bajir. The Ssstram are capable of speech, and are a tribal race. Unfortunately, they are NOT on the top of the food chain on their planet; they are hunted and picked off by far stronger creatures--probably not sentient. So the technologically advanced Yeerks offered protection to the Ssstram, in exchange for their bodies. The Ssstram's small numbers, and their desire to advance in technology and population, allowed the Yeerks to take the entire race.
But the Ssstram are unable to breathe the same air as humans, or Andalites, or Hork-Bajir. Their planet's atmosphere is made up different elements. So the Ssstram--though suitable hosts-- are unable to help the Yeerks expand their Empire.
