I accidently told my MacBook Pro to forget a Bluetooth device. I have no idea how to bring it back. How do I bring it back?
Using "System Preferences" -> "Bluetooth" you should be able to add or "pair" the device in question as long as the device is in range and broadcasting a signal. You might need to turn the device off then on again, or tell it to "pair" or "repair" before it is visible from the MacBook.
Bluetooth devices usually have "Pairing Mode".
You may have to put the device on pairing mode to be shown again in devices list in Bluetooth preferences and be able to pair it. In headphones it is usually done by turning the device off and turning it on again by holding the power button for a certain amount of seconds.
Check your device's catalog. You'll definitely find it there.