In this case, the growth of a wild horse population is most likely limited by density-dependent factor.
Here, if we compare the numbers in both the cases, we can see that 1/4th of the population is killed in both the cases, which depends on the overall number of the herd. In the second case, the overall number of horses in the herd at the start is double that in the first case, but it leads to a similar ratio of horses being killed due to the particular change.
Since they are said to live in equal territories and thus, the second herd has double the density of that of the first case, and it leads to double the deaths as well. Thus, the factor that limits their population growth kills individuals proportionally to the density, thus leading to it being density dependent.
