Presumptions Are the [Name-the-Animal]s of the Law

A line of cases, from Mackowik v. Kansas City, St. J. & C.B.R. Co., 94 S.W. 256 (Mo. 1906), to an opinion handed today by a Washington state appellate court, describes presumptions as a particular animal of the law.

What animal are presumptions supposedly like, and why? No peeking.