Use a big pair of channellocks to take it out. Most trailer repair shops have them or can get them. Probably would be good idea to replace all of them, keep an old one for a spare.
It is a screw in cap, not sure of the size but it looks like a Fruehauf/Propar cap.
The cap had a large nut cast into it.
It is probably polycarbonate, I would try to just break it out or hit with a chisel and unscrew it.
Here is what the other 3 hubs look like. They are rough looking but the trailer is used to haul square bales and hasn't been over 5 miles an hour in the last 15 years.
I spent like 20 bucks to replace all of them on my trailers and To have spares. Cheap insurance. You need to do something and make sure there’s oil in there