As far as I'm aware (and this is nothing official, just geeky forum talk) there are devices that claim to be mp5 players, but mp5 is not actually a video format in the same way mp4 is.

I think it could be that an mp5 player does not require videos to be converted into a mp4 format to be played, as is required on an mp4 player. Or it could be just a marketing ploy.

Either way, no well known brands are making mp5 players, so if you do buy one it will be cheap and rubbish. I don't personally see the point in them.