Pliny the Elder, the Roman military commander and naturalist, has this to say about the birth and character of puppies (catch the reference to dogs likely to suffer nightmares!):
CHAP. 62.—THE GENERATION OF THE DOG.
This animal brings forth twice in the year; it is capable of bearing young when a year old, and gestation continues for sixty days. The young ones are born blind, and the greater the supply of nourishment from the mother's milk, the more slowly do they acquire their sight; still, however, this never takes place later than the twentieth day, or earlier than the seventh. It is said by some writers, that if only one is born, it is able to see on the ninth day; and that if there are two, they begin to see on the tenth, every additional one causing the power of seeing to come a day later. It is said, too, that the females which are produced by the mother in her first litter, are subject to the night-mare. The best dog of the litter is the one which is last in obtaining its sight, or else the one which the mother carries first into her bed.
-- The Natural History of Pliny the Elder; see text and footnotes of this chapter here, which will also permit you access to the entire translation.