This was in a scrimshaw kit that was purchased at a yard sale about 20 years ago. Kit was sold by a company on Nantucket, and was old at the time of the yard sale. It is not known if this and the three other similar pieces were original to the kit or were added later on. What animal did this come from?
From the location of purchase, conical shape, color, and size, the tooth is that of a sperm whale (Physeter macrocephalus). Sperm whales have 17-29 teeth in the lower jaw that range 3-8 inches (see Perrin et al., 2009). Their teeth were most often used as scrimshaw, and many can be seen at the Nantucket Whaling Museum.
Just as an FYI, under the Endangered Species Act, it is illegal to buy or sell sperm whale ivory harvested after 1973. So you may want to have the tooth aged.