por·poise 
(pôrpəs)
(pôrpəs)n. pl. porpoise or por·pois·es
1. Any of various marine toothed whales of the genus Phocoena and related genera, characteristically having a blunt snout and a triangular dorsal fin. Porpoises are placed either in their own family, Phocoenidae, or with the dolphins in the family Delphinidae.
2. Any of several related aquatic mammals, such as the dolphins.
[Middle English porpeis, from Old French (probably translation of a Germanic compound meaning sea-pig) : porc, pig (from Latin porcus; see porko- in the Appendix of Indo-European roots) + peis, fish (from Latin piscis).]