For the story that I am writing currently, it is set on a colonized planet thousands of years after machines wiped out most of the human colony that were established there, and would go on to form an ecosystem with other organisms since they depend on biomatter as fuel. The machines themselves have evolved to take on the forms of predators such as big cats or theropod dinosaurs and ultimately have driven most other apex predators to extinction.
My question is what would be the largest possible organism to exist in an ecosystem with these types of predators. The ecosystem mostly comprises of herbivorous machines but these predatory ones need prey themselves. What types of animals would be large enough to cleanse the hunger of let's say a tyrannosaurus sized machine without going into direct competition with possibly more efficient plant-eating machines?
The planet itself was terraformed through a combination of future tech and ancient magic to make it as earth-like as possible. The planet as a result has numerous dynamic biomes ranging from forests, swamps, tundras, deserts, etc. Each of these ecosystems would be occupied by different types of machines, speciallized to specific lifestyles and options for resources.
The robo-herbivores that the herbivorous animals would be competing with would be different based on each biome as a result of this. Certain machines would adapt to browsing or grazing specific foods, and would have to niche partition in order for the organic herbivores to survive.
To be specific, I'm talking about grassland. I wanted the humans in this world to have horses as mounts but not sure if that's unrealistic.
The animals would be competing with a number of robo-herbivores that have adapted to life on the grassland. Many of the machines resembled hadrosaurid/iguanodont dinosaurs in their physique and behavior as herding animals, each species only being separated by food preference (different grasses and bushes) and appearance. These robot-herbivores are around 20-30 feet in length and about 1 ton each. There are also large bulky animals similar to elephants and ceratopsians with each individual possessing a long tentacle like appendage designed for grass collection and browsing. These measure around 50 feet long and 3 tons. Although all these species feed off of mostly the same kinds of plants, they tend to feed at different times and different regions of the grassland.
Our largest organic species (probably an herbivore) would need to partition in similar ways to these robo-machines to avoid direct competition.