Years ago my groom/helper for the day, whose blushes I will spare in this instance, appeared and said in a very puzzled voice: “I’ve just seen David Green walking the XC with a unicycle.”
I’ll admit it took me a few seconds to work out what on earth she was going on about. At which point I nearly wet myself laughing.
Answers/Guesses below:

Unicorn, obviously…
A trundle wheel? Metre measure.
My favourite recent one – at about 2am in the lorry park at Barbury:
“Just because I don’t want to have sex with you it doesn’t mean I’m gay”
Worth being woken up for I thought!!!
A measuring wheel!!
Yes, it was a measuring/trundle wheel.
Fizz, that’s just hysterical. WHO?!?!?! 😉
My favourite recently was a pony club mum who’s darling offspring had had a brilliant dressage, a couple of fences SJ & a terrible XC round. She was overheard at the scoreboard moaning because they hadn’t been allowed to school around the XC course before the competition and;
”What’s the point of a 28 dressage if the jumping lets you down’
[snort] I’m sure the last time I looked that was the whole point of eventing 😀