I tried my best to link some relatively mysterious creatures.
My Theory:
The Old Ones caught early Zoats already existing on the whfb world to modify; uncaptured Zoats retreated to the forests to stay hidden.
The captured Zoats began being genetically modified into other creatures, two of which are Dragon Ogres and Troglodytes (a type of primitive lizardman); populations of these escaped into the wild or were discarded to govern themselves.
The Old Ones created "true" Ogres and Lizardmen, potentially from this stock or otherwise—the origin remains truly unknown. Evidence for this hypothesis is the morphological traits shared by different species, combined with the knowledge of how the Old Ones genetically/magically modified and created many of the known races.
Each population throughout this development had a purpose:
Group A: Zoats -> Troglodytes -> [potentially] Lizardmen?. They were modified from being quadrupedal and were "dumbed down" to eventually be remade into a group absolutely subservient to the Old Ones as chief servants, enforcers, and builders.
Group B: Zoats -> Dragon Ogres -> [true] Ogres. They were modified over this process to be highly adaptable laborers in climates where Lizardmen were less able to thrive, before shifting to an anti-chaos role. The interim most of the Dragon Ogres were made to resistant to further modification for greater adaptability; the bulk of them were discarded into the world where they fell to Chaos en masse, fearing their former captors. They remain quadrupedal and reptilian on the bottom half.
So many thousands of years have gone by that all five extant populations have forgotten their related history.
Anyways that would explain why so many different similar things exist at least on part including why radical different traits are present in similar but not known to be related things.