I believe the thesis in this is actually the first sentence considering the rest of the story talks about that first sentence.
Answer:
We can infer that Zlateh was still a productive animal, capable of promoting Aaron's survival.
Explanation:
When reading the short story we can see that Zlateh was a very dear goat for the family that raised her, but due to Zlateh's financial difficulties and aging, the family decided to sell it to a butcher, because she believed she was no longer more as productive as before. Aaron, was the boy responsible for taking Zlateh to the city to be sold, but due to environmental conditions Aaron and Zlateh are trapped in a haystack.
Although Aaron had no food to survive while he was still worth it Zlateh ate the hay to produce milk and keep the boy alive, proving that he was still productive and useful to the family.
The topic sentence of each paragraph in an essay should support your thesis. Your thesis states the reason for writing the essay, so it is vital to include supporting details in order for your thesis to be considered valid.