'The mickeys are slipped' I think is more likely to refer to a 'mickey finn' which is a drugged drink - see https://en.wikipedia.org/wiki/Mickey_Finn_%28drugs%29 - I would surmise that the opposing horses - or at least those of them considered likely to beat our horse - have been given (or 'slipped') drugs to slow them down.
'The mickeys are slipped' I think is more likely to refer to a 'mickey finn' which is a drugged drink - see https://en.wikipedia.org/wiki/Mickey_Finn_%28drugs%29 - I would surmise that the opposing horses - or at least those of them considered likely to beat our horse - have been given (or 'slipped') drugs to slow them down.