Ascended To Carnivorism: The horrane and the raboon are top predators descended from monkeys. Likewise the predator rats fill the same niche as present-day canines.
Australian Wildlife: Australia has collided with Southeast Asia & become a rainforest continent. Among others, there's the giantala, a giant lumbering kangaroo, the posset, a "marsupial pig" of sorts, and the slobber, an Eyeless Face creature that catches insects with its salivation.
Backup Bluff: Threatened by birds, the Terratail rodent ducks behind a branch, hisses, and sticks its long tail (which resembles a snake) in its predators' faces.
Bat out of Hell: There's a newly formed Pacific archipelago inhabited by various strange species of flightless bats. Probably the least scientifically plausible of the creatures presented (there could be flightless bats, but it'd be unlikely they'd produce forms like the nightstalker). That said, at least they inspiredPrimeval's "Future Predator" and Subulba from Star Wars Episode One: The Phantom Menace.
Bizarre Sexual Dimorphism: The Matriarch Tinamou (female is similar to an adult turkey; male lives as a wren-like symbiont that rides around on her back), the predatory Bardelot (male looks like a polar bear, female is a huge, Bad Ass saber-toothed beast) and the Common Pine Chuck (female resembles living songbird, male has a massive beak for crushing seeds & nuts) are the three weirdest examples.
Chest Monster: The oakleaf toad lures in prey with its worm-like tongue, while both a bird and a bat mimic flowers to attract insects.
Everything's Better with Penguins: Giant, marine descendants of modern penguins took over the place of whales. Anatomically speaking, there are a couple of problems (namely, the flexibility of the spine and the vivipary thing), but otherwise these birds are probably among the most accurate creatures from the book. Which is really saying something.
Expy: Many of the life forms are Expies of real ones, both living and extinct. The text Lampshades these as examples of convergent evolution.
Feathered Fiend: There are several predatory birds, only one of which seems to be related to any modern birds of prey.
Maniac Monkeys: The cheetah-like Horrane and the theropod-like Raboons.
While not true predators, the Khiffah sometimes leads a foe into a trap, and then eats it.
The swimming monkey is a hunter, albeit of fish rather than mammals.
Messy Pig: Various species and even entirely new families of herbivores evolve from them. Most are pretty believable. The Turmi, an anteater-like pig, less so - it would be implausible for it to survive for long, since pigs would have to eat a lot of termites to sustain themselves. It would have to keep continuously eating and not move at all.
Still possible, as it's never said how large the turmi is supposed to be, so it might be the same size as the Real Life aardvarks or anteaters that get by just fine that way.
Noun Verber: Many of the animal names follow this trope.
Nutty Squirrel: The chirit, which has a long, tubular body it can twist in virtually any direction. There's also a squirrel with a spiked tail for use against predators.
Panthera Awesome: The striger, the last of the felines, and the first predator in Earth's history to develop adaptations specifically for preying on monkeys and apes.
Portmanteau: Some of the animals are named like this, such as the rabbuck (a lagomorph that's taken over the deer ecological niche: rabbit + buck), and the shrock (a large, black-and-white striped insectivore-descendant: shrew + brock)
Rodents of Unusual Size: Rodents are, in fact, the dominant predators of the new world (despite many other more plausible candidates, like shrews). Some weird things like the aquatic, hippo-like Mudgulper or the kangaroo-like desert leaper are present as well.
Rodents in South America didn't turn predatory, but did evolve into larger forms like the tapimus, strick, and wakka.
Science Marches On: A lot of the science was already shaky in the 80s and thirty years of scientific advancement haven't done it any favors.
Spiritual Successor : The 2003 TV series (and companion book) The Future Is Wild, produced by Animal Planet, takes one step further and shows three different future eras of life on Earth : 5 million AD, 100 million AD and 200 million AD.
Time Passes Montage: The illustrations of savannah predators include three similar views of the same dead gigantelope, being fed upon in turn by horranes, raboons and gholes, until nothing is left but bones.
Toothy Bird: Not exactly "toothy", but there's a kingfisher descendant with tooth-like serrations on the beak.
X Meets Y: A huge number of the animals are basically crosses between two modern-day animals: