High intensity Najasaurus's are great tames. I'm not sure what server you are on so that can affect price/demand greatly.
Platinum/Crimson drakes are cool pets if you get high spawn intensity. People like to collect one of each kind: Fire, Cold, Energy, Poison, Physical.
Also right now the Krampus event is spawning Frost Drakes which even at low intensities are equivalent to very high intensity Platinum/Crimson Drakes.
As stated above special hue Cu's can be worth a ton, even on low population shards.
Tritons are a pet you cannot tame but can get from doing Pirate hunting and Beacon destruction. High intensity ones are very hard to get.
If you have an pets in your stable such as: Pre-Patch Nightmares, Pre-Patch White Wyrms, or Bane Dragons (these no longer spawn), these are rare pets and sell for a lot.