...and for those that are wondering, MANTA is short for mantaray which is the codename for the Nexus 10.
https://developers.google.com/android/nexus/images
You got it exactly.I see how JAPetty got there by accident as the same thing just happened to me. Like me he has a universal case with rubber bands on each corner to hold the tablet. The band on the top left corner can foul the on/off switch and the volume control toggle sometimes at the same time and that is what happened to me. I am new to Android and only just recently bought the Nexus 10 so my question is having downloaded this Manta 8 in error should I keep it or uninstall it?
You didn't download Manta 8. As someone previously stated that is a code name for the device. Once the download starts I would not kill it. Just let it finish and follow any instructions.