I think that’d be “Spitting Image”.
Well done, Tygra_Rules! That is the correct answer. Nayda has a small cameo in it when the fake Panthro chases her and uproots the tree she is hiding in.
Your turn, sir! 
OK, here goes an easy one. Cheetara and Tygra appear in every season one episode but two each. In which episodes do they not appear?
Easy indeed!
Cheetara is absent from “The Mountain” and “The Superpower Potion” (indeed so are the kittens - and indeed Lynne Lipton is not present at either recording)
Tygra is absent from “The Terror Of Hammerhand” and “The Trial Of Cunning” - and although he is seen in “Tight Squeeze”, he has no lines.
You’re right, Liam! Your turn!
I think neither of them (at least Tygra, can’t remember if Cheetara as well… now I’m not sure about Tygra either lol) have any lines in “The trouble with Thunderkittens”, when they still the TCs’ weapons while they are in a space mission.
Tygra has one whole word of dialogue in that episode, “oh?” in the final scene. Cheetara and Panthro have few lines - Cheetara seems to be in charge of the mission up there in the Feliner.
In any event, my question:
Besides Tygra of course, who else became invisible in the series?
Nayda in “Out of Sight” springs to mind, but I feel like there’s maybe one other?
Besides Nayda (as Zax correctly pointed out), Monkian turned invisible in “Monkian’s Bargain”.
Oh! Not sure if it counts, but Grune is invisible when first interacting with the ThunderCats in “The Ghost Warrior”.
LOL I also thought of that one, but wasn’t sure too if that would count.
Bump!
Oops! I’d forgotten whose question it was. I can’t remember if I’d made a list of all the answers to this one, but as Zax has got the most already and it’s been a while, I say your turn sir!
Okay, here’s a new one. Not too hard, I don’t think.
In what episode does the Sword of Omens get pulled from a stone?
Hint: It’s not “Excalibur”.
I believe in “Ravage Island” Lynx-O pulls it out of a rock (stone). Also (and I’m not sure if this counts), in “Garden of Delights”, the Sword frees itself from Mumm-Ra’s grips and gets stuck on his sarcophagus (which is also made of stone). Then Mumm-Ra pulls it out.
I think also in “Queen of 8 Legs”, Tygra pulls it out froma stone before throwing it at Snarf.
I guess there were more stones than I was remembering! “Ravage Island” was the one I had in mind, and Wilycub did offer a second option. Winning points to Wilycub!
Thank you, Zax! Ok, here’s mine:
How many seas were there on Old Thundera?
Twelve?
That is correct, Liam! In “All That Glitters”, after Tygra cuts the Sword of Omens in two and realizes that he is fighting Lion-O, he exclaims, “By the twelve seas of Thundera!”
Your turn, sir!
OK, here’s one.
What is the oldest trick in the book?