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
1948 Allis UC Hi-Crop Cane 1941 Allis WC 1948 Allis G with loader, 1948 Allis G with dual disc 1948 Allis G 1948 Allis G 1949 Allis G 1954 Allis G 1959 Ferguson TO-35 1972 MF 135 1979 Power King 2418 1989 Ford 345C TLB
Clean off one of the good ones, there will be a number cast into the plastic and usually a brand name.
If you get the info, an big truck parts place like Truckpro or Fleetpride will have them