They aren't bright and all they want to do is "Dig, dig, dig. Burrow, burrow, burrow." They won't hurt you unless they think you pose a threat. If you do not pose a threat, they just ignore you.
Moss Man of New Thundera
This mossy giant was awakened by the force of the Feliner's engines while Tygra was trying to lift off the ground. Angered about having its snooze disturbed, it started attacking everything that moved. It could regrow severed limbs and shoot eye blasts that cover everything in moss.
Demolisher
His goal in life is to fight. He fights for no reason, except to win. He fights anyone he deems to be a worthy oponent. He doesn't care if he fights good or bad, just as long as he fights somebody.
Dirge
His goal in life is to beat a drum and chant things like "Demolisher wins." He goes where the Demolisher goes, for he is the Demolisher's sidekick.
Char
This guy is a blacksmith / scrap metal collector. Sometimes he helps the Thundercats and sometimes he hinders them. He does not care to fight evil, but is not necessarily evil. That's why he's in this section.
Quick Pick
Is a master pickpocket. He is reforming, but progress is slow.
Char-Nin
He's a genie, except he is trapped in a harp instead of a lamp. Like all genies, he wants to be free. Even though he started out as a good genie that wanted to grant wishes, he turned bad (like expired milk) when Mumm-Ra promised to free him if he would destroy the Thundercats.
Char-Nin's Harp
While most genies have a lamp, Char-Nin has a harp. He is bound to it and longs to be free from it. Whenever he is inside it, he uses the bust on the harp to communicate. The harp can be activated by playing it.
The Caveman
Not exactly good or bad. He has a simple code of honor (which is good because he is too dumb to handle a complicated one).
Under-Earth People
Their ancestors were forced to live in underground caves. Over time, their eyes grew accustomed to the dark, but they can no longer bear bright lights. They have a great library, but are unable to read the books because enough light to read by would damage their eyes. They force anyone who comes into their territory to read to them. Liono became friends with them after saving their books. I am uncertain if they have completely changed their ways though (so I am putting them in this section).
Ice King
He was once trapped in ice. When he was freed from his icey prison, he went to find a magic egg in Snowman's castle. He caused a great deal of trouble getting the egg. He didn't really want to hurt anyone, but he froze anyone that got in his way. He's currently trapped inside the egg, probably for all eternity.
Tabots
Little is known about this species. This particular one is implied to be greedy. It is also possible that they may have ties to the baddies, though this is only hinted at in The Thunder Cutter. They are a pig-like species and make piggy sounds while talking.
Ezuka
Ezuka is the water serpent of Dreary Canal, on new Thundera. He really likes water and is can spit out a web-like substance.