Here's another thought to ponder.
Biology is also a hard science. An animal encounter should not be random, as such. On an arctic tundra, characters could encounter a massive herd of mobile grazers, bulky with long, shaggy hair to insulate against heat loss. Such herds are likely to attract pack hunters like wolves chasing down caribou, moving alongside the herd, harassing the herbivores, picking off the slow, the weak and the lame.
A rainforest environment generates a huge biodiversity: insect swarms, packs of arboreal primate-analogs hurling large fruits and less pleasant substances down on the heads of incautious adventurers, strange plants that stink of rotting meat, bugs, parasites and large, solitary pouncer and trapper predators.
A planet with a low gravity and a dense atmosphere is likely to produce a lot of flying species; a world with high gravity is unlikely to produce flying species, and will have ground-dwelling creatures and plants which evolved to be short with broad canopies, strongly structurally reinforced.
All these animal encounters are the product of their worlds; their presence is the result of evolution producing adaptations in the genomes to produce animals and plants which are perfectly suited to their environment.
They're not just there because some random dice roll says they're there, like finding a dragon in a room with no large exits in some dungeon crawl.
And where there is evolution, of course there are the failed species, for which there will be a fossil record of some sort.