Toxicity: 30–60 mg (0.5–1.0 mg/kg) can be a lethal dosage for adult humans.
As a rough estimate, a horse can be 400kg, so a lethal does of 200mg-400mg for a horse. Density from wikipedia 1.01 g/cm³.
This means a lethal dose would take up 0.25-0.5 cm³ which is a drop of diameter 0.8-1 cm. This is quite a large drop, approximately 5-10times the dose you stated (0.05ml), but isn't too far from being a drop.
If horses are particularly susceptible to nicotine, then it could well be a lethal dose at 0.05ml.
Edit: This puts the LD50 (The dose where half the subjects die) in horses as 100-300 mg/animal, so if 2.5 0.05ml drops kill 50% of horses, its possible that 1 drop could kill a horse, even if it didn't kill most horses.