For a 20 gallon, I know a few oddballs, not all are going to look super prehistoric.
Your best bets for a prehistoric looking fish are these, though some require extremely advanced care: Thalassophryne Amazonica
Also Monocirrhus polyacanthus, Carinotetraodonlorteti, mottled sculpin, African muskipper, African freshwater pipefish, banjo catfish, African leaf fish, Dwarf Climbing Perch and potentially an African butterfly fish. If you're going to get a 20 gallon, go for a 20 long
Some oddballs:
Wrestling halfbeak
Wimple piranha
Dwarf Pike Cichlid
Compressiceps Cichlid