Difficult one! I have a lovely Irish cob who is sweetness and light 90% of the time, but has a huge sense of humour and will throw in an occasional wicked buck - however I see this as 'self expression' as is never aimed at getting me off!
I think there are more 'nasty' horses out there than naughty, in terms of being aggressive, rearing and pushing their owners around etc.
Naughty - that's when my grey comes up for a hug, sees that I have a headcollar with me and sods off up to the top of the hill with her tail in the air! If I catch a different horse (particually the gelding) she comes galloping back down again and wants to come in too.
hmmm mostly i would call it cheekiness or character but we did have a naughty greedy pony who would do his utmost to escape from any diet paddock you put him in no amount of electric tape would stop him, he would pace up and down the fence assessing it and then walk away, turn and charge at it like a goat with his head down! he learnt that if he ran fast enough at the gate hooks the spring would stretch and he could get through it was a good job my daughter loved him as he was threatened with the glue factory many a time as i retrieved him from various parts of the farm
I always call our two naughty, even though mines anything but & daughters is cheeky more than anything, eg naughty pony tipping the wheelbarrow. So more a term of endearment!
For a badly behaved one I'd be more likely to describe it as bad mannered, badly schooled, needs work etc. Being a bit too cheeky I'd just say pushing its luck or stroppy mare, whatever seemed most accurate.