en.wikipedia.org/wiki/Yahoo_(Gulliver%27s_Travels)

The claim is inaccurate in two ways:

1. They are not “legendary.” Yahoos are fictional beings created for a novel, not figures from legend.
2. They are not simply “human beings.” Reference works describe them as an imaginary race of brutish creatures. Oxford’s learner’s dictionary says: “The Yahoos are an imaginary race of creatures in Gulliver's Travels … They are animals with human form,” and gives the word origin as “the name of an imaginary race of brutish creatures in Swift's Gulliver's Travels (1726).” Project Gutenberg’s catalog summary likewise describes Gulliver encountering “rational horses living among savage human-like creatures.” Swift’s own text repeatedly calls Yahoos a species of animals and brutes.

So the sentence should not identify them as “legendary human beings.”

The problem here is the date attached to the letter. David Berkowitz’s shootings began in July 1976, but the specific police letter that used the term “yahoos” was not from 1976.

CBS News’ timeline states that on April 17, 1977, Valentina Suriani and Alexander Esau were killed and that “Berkowitz left a letter at the scene, signed ‘Son of Sam.’” The Crime Museum’s Berkowitz biography is even more specific: at the April 17, 1977 crime scene, police found a handwritten letter addressed to the NYPD captain, and in that letter Berkowitz called himself the Son of Sam.

So while Berkowitz was already committing murders in 1976, the statement is incorrect as written because the letter to police mentioning “yahoos” was from 1977, not 1976.