One or more of the characters is a Time Lord.Specifically, Holly is the Rani. In the episode "Elsewhen", Holly encounters a blonde woman apparently in her twenties who calls herself "Rani", who turns out to be Holly's older self come back in time to help her. Rani admits that she can time travel at will and understands the technology of the Land. Given that the pylons of the Land display TARDIS-like properties, it seems reasonably plausible that at least one is a TARDIS, and the future Holly used it to escape the Land to Gallifrey where she, like Ace would have in the original series, became a Time Lady. How she then went from girl hero to Mad Scientist (and from blonde to a brunette) can probably be attributed to a bad series of regenerations.
The Sleestaks did not evolve naturallyThey were constructed, just like the rest of the Land, by the ancient humanoids seen in "The Musician." Then the ancient humanoids went about transplanting prehistoric life, including the Dinosaurs and the Pakuni, from different time periods on Earth to the Land. Maybe the goal was to test whether an intelligently designed species could compete against naturally evolved predators.
The Sleestak God is a Great Old One...making the series an adjunct to the Cthulhu Mythos. The depiction of the Sleestak God as an unseen, but apparently massively large, insatiably hungry mass deep within a network of caves and attended by a race of brutal, mindless servants is rather Lovecraftian. Some fan theories have linked it to Tsathoggua, a creation of Clark Ashton Smith for his Hyperborean stories within the mythos Shared Universe, who fits most of the above descriptions quite well, with the exception that its cult is said to be hairy and ape-like, while the Sleestak clearly aren't. But who's going to tell it that it can't have more than one cult?