Entry from US Dictionary
1950s: named after Norman J. Holter (1914–83), the American biophysicist who invented it
When a patient is fitted with a Holter monitor, the recording device may be on a strap that goes around the neck or over the shoulder. This procedure is so suggestive of wearing a halter that it is not surprising that many people, including some healthcare professionals, believe they are correct in calling the device a halter monitor. In fact, the true name has nothing to do with the way the device is worn, but refers only to the man who invented it: Norman J. Holter. Even though the pronunciations of Holter and halter differ, the mistaken identity is persistent.