Final answer:
The term 'dysmaturity' refers to a foal that has had a gestation period significantly longer than the average, such as 400 days, and may show signs of underdevelopment.
Step-by-step explanation:
The term that usually refers to a large foal that experienced a longer-than-expected gestation period, for example, 400 days, is dysmaturity. Gestation periods indicate the length of time from conception until birth in viviparous animals. While the average gestation for horses is typically around 340 days, cases in which the gestation period significantly exceeds the average can result in a foal being characterized as dysmature. This term suggests that, despite the longer gestation, the foal may have characteristics of being underdeveloped.