If you have a horse who doesn't like having his rug on, then equine behaviourist Jill Butterworth has a few clues as to why.
Q: My horse puts his ears back every time I put a rug on him. Why might this be?
A: There are several reasons why he may do this.
The first thing to do is check that the inside of his rug is clean, for example, with no clumps of itchy, matted hair or dirt, and ensure that you use hypoallergenic products to clean his rugs.
Also, check his skin thoroughly for signs of irritation, redness, sores and scabs etc. and ask your vet to examine him if you’re worried.
If he also reacts like this to his numnah or his saddle, ask a qualified saddle fitter to check his tack.
However, if he doesn't show signs of tension at any other time relating to his back, and isn't exhibiting avoidance behaviour, then you could use a technique called counter conditioning and introduce a small treat thrown into a bucket when you bring his rug out.
Then, when you've put the rug on, throw another small treat into the bucket. He'll soon realise that the rug brings a reward and he'll actually look forward to it.
I advise wearing a hard hat, as well as gloves and boots, for any handling activities with horses as, occasionally, they act unpredictably.