I enquired about a fairly cheap horse for sale and the owner got back to me saying that he was sound but had "fluid in the knee". She said he cut his knee months ago but the leg was still fluid-y. The vet has apparently come out and said the horse should be fine. She said as they have just pretty much had him out at pasture so they haven't done anything about the knee.
I've seen horses have swollen joints with fluid in them, but generally these have gone away in a few days.
If I were to be interested in this horse I'd get a vet check but is he even worth looking at? I'm not quite sure what this condition is/what causes it - is it one of those things that can be fixed or not? Is it something that will cause trouble?