In a mare with one small ovary and the other fluid-filled, what is the diagnosis?

The diagnosis of a granulosa cell tumor is supported by the presence of one small ovary and the other fluid-filled ovary in this mare. Granulosa cell tumors are the most common type of ovarian neoplasia in mares and can lead to the enlargement of one ovary, while the other may atrophy due to hormonal imbalances caused by the tumor.

These tumors are typically characterized by the production of excessive amounts of estrogen, which can disrupt normal reproductive functions and lead to signs such as irregular estrous cycles or even estrus behavior throughout the year. The fluid-filled appearance of the affected ovary may correlate with the tumor's cystic characteristics or changes in hormone levels resulting from the tumor's secretion.

In contrast, ovarian cysts and follicular cysts commonly lead to both ovaries being affected or do not present with the pronounced atrophy of one ovary. Uterine infection would not explain the specific ovarian findings mentioned. Thus, the combination of a small ovary and a fluid-filled ovary points toward a granulosa cell tumor as the most fitting diagnosis.