"small, active dog used to kill rats," by 1852, American English, from rat (n.) + terrier.

"a rattling sound or effect," 1680s, echoic, originally of a cooper hammering tubs.

"one whose business is the catching of rats, a ratter," 1590s, from rat (n.) + catcher.

also rat's-tail, from rat (n.) + tail (n.1). Used since 16c. of conditions, growths, or devices held to resemble a rat's long, hairless tail in any way, including "lank lock of hair" (1810); "end of a rope" (1867). Related: Rat-tailed. A rat-tail file (1744) is a fine, round file used for enlarging holes in metal.

"snake which kills rats," 1818, from rat (n.) + snake (n.).

also rathole, "the hole gnawed in woodwork, etc., by a rat or rats," 1812 first in the figurative sense of "nasty, messy place;" from rat (n.) + hole (n.). As "bottomless hole" (especially one where money goes) by 1921.

1951, "a gang of disorderly young people" [OED], from rat (n.) + pack (n.). In reference to the Hollywood circle around Frank Sinatra, from 1958.

"trap for catching rats," late 15c., rat trappe, from rat (n.) + trap (n.).

"something used to poison rats with," especially arsenic, 1799, from rat (n.) + poison (n.).