Any ideas?

That looks like the one my mare has. In her case we have decided it is similar to a windgall and caused by having a turned in foot that side. Same leg too.
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Thankyou for the trouble you’ve taken to answer. My horse is in regular full work, went on hound exercise last Thursday, fun ride on Sunday, day off Monday and then on Tuesday I thought he may look a bit more swollen on that leg but still sound! I’ve popped him out and will field rest him till vets seen him on Friday. I’m worried it’s a tendon threatening..he’s 16 . Here’s a pic from last night before cold hosing and bute
 
It does look very similar to my mare. Will be very interested in what your vet says. Hopefully nothing serious. Mine is retired so it doesn't matter in the grand scheme of things. It doesn't hurt her and she isn't lame on that leg.
 
Looks 'windgall' like but higher in the leg than normal, but if the horse isn't lame that can only be a good thing. Keep us informed!
 
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