As silly as "Pi-pipipupu" sounds, it is a fairly accurate onomatopeia on rodent squeaks.
It may seem somewhat unrealistic that so many of the hamster descendants end up looking so much like modern animals but remember that modern mammals evolved from creatures that resembled modern day rodents, and HP-02017 was specifically terraformed to be like Earth. A similar starting point in similar conditions would likely lead to many similar end results. The one notable exception is their teeth, as rodents have specialized dentition, which carries over in their descendants as far removed from traditional rodents as imaginable.
On a similar note, its also less implausible for the lizard-like rattiles to come from mammals than somebody might think when one remembers that multiple different groups of unrelated reptile have evolved to have the highly versatile lizard bodytype. The likely reason we haven't seen a mammal take on the lizard niche in real life is that it would have just been quickly filled by a different reptile while HP-02017 has no reptiles at all.
Indeed there is precedent for mammals losing their endothermy - Myotragus, a species of goat that evolved a metabolism similar to a reptile. With no other cold-blooded vertebrates to compete against, it makes sense that aberrant cold-blooded hamster species would inevitably appear and outcompete their endothermic relatives in niches where a slow metabolism is more advantageous than a fast one.
It is even possible that all amniotes were ancestrally endothermic so what happened to ratiles might very well be the case for all reptiles from our world.
The canine-like zingos and the convergent Isla Centralis scabbers in particular are similar to the very-repetitive trend of unrelated cursorial carnivores evolving into dog-like forms. This has happened at least six times in mammals (true dogs, hyenas, thylacines, amphicyonids, mesonychids and creodonts. Some extinct opossums, stem-kangaroos and Mesozoic mammals may also count), several times in non-mammalian synapsids (cynodonts, therocephalians and gorgonopsids) and even in an archosaur with the terrestrial crocodilian Araripesuchus, also known as the "Dogcroc".
Then there's the fact that many unrelated animals that developed heads that resemble those of canines like the skewering harpshrike or the ring-necked wolfface. Such a skull shape is fairly easy to develop for mammalian carnivores while human-like faces are nearly unheard-of on Earth animals as its far more specialized.
The rather familiar-looking siege of the Bruteriders and their rakatusks happens in Mesoterra. And what does "Mesoterra" translate to? Middle earth.
Many of the mammals on HP-02017 are noticeably more colorful than Earth mammals, with colors and patterns of red, bright yellow and orange being fairly common for them while being rare for mammals on Earth. The difference is that HP-02017 has a second sun that regularly bathes the landscape in red light, leading many animals on the planet to develop the ability to see into the red spectrum. Something pretty rare for Earth mammals outside of primates.
The harmsters and 'hounds aren't one species, but several, despite Tribbetherium repeatedly emphasising that sapience is rare. This may be a parallel to how there used to be multiple human species that were presumably as intelligent as us.
The ratavult is a vulture-like scavenger but unlike vultures, only its snout is hairless while the rest of its head and neck are still fully furred. The major difference between their skulls is that the ratavult has teeth while the vulture does not. Vultures have fairly weak bills despite their scavenger lifestyle which forces them to stick their head all the way inside a carcass in order to acquire any good meat, necessitating the loss of feathering to keep rotting viscera from sticking to them. A ratavult's teeth means that even if they have weak jaw muscles, they can still easily remove strips of flesh from the outside of a body, meaning their mouth is the only thing that'll get dirty.
It's mentioned that the SIHTT infection can prolong the life of the inflicted Harmster by several months before it eventually kills them. It sounds horrible enough on its own but remember that the Harmster natural lifespan is so short that fifteen is considered old. From their own perspective, they are left in that horrific state for years before they finally die.
It's revealed that in the Temperocene the transmissible tumor that gave rise to SIHTT has since evolved into a free-living fungus-like organism known as the shroomor. Which means that, on a cellular point of view, the Harmsters are still an extant species: only now taking Formerly Sapient Species to an even more horrifying level than the Brutes.
, a species of goat that evolved a metabolism similar to a reptile. With no other cold-blooded vertebrates to compete against, it makes sense that aberrant cold-blooded hamster species would inevitably appear and outcompete their endothermic relatives in niches where a slow metabolism is more advantageous than a fast one.