So... this was my prompt:

Guinea Pig Clan....
The.
Caustic (being sarcastic but in more of a bitter way)
"Oh, wow, I just CANT WAIT to collect moss. It'll be. SO. FUN."

And popular (I hope you know what popular means)

Although... 'Popular' in the prompt isn't exactly going to work out well because they're going to have no competition.

Guinea Pig Clan first began by a cat named Binks, whom ran away from home. She then, gathered all of her neighborhood cats and convinced them to run away towards the forest with her, and, surprisingly, most agreed.
Then a magical cat named LeafStar just... popped next to them out of the blue and began to teach them how to become Warriors. Now after she teaches them the basics she told them to come join her clan, LeafClan, but Binks refused by pointing out how skinny they were, and that if they went with them, they would die.

Most agreed with Binks and followed her towards an unknown land, and Binks renamed herself BinksStar.
They settled in a jungle/forest with many cliffs and snakes.

Now, you might be wondering, WeLLl, OkAY PERsOn, how did the Guinea Pigs come into play??

Ah, good question!
You see, when they settled there, two days later the old Leader before LeafStar, SpiderStar, told them that they must choose a Clan name in order to become a true clan, and to worship and care for their ancestors.
Or course, this changed the clan in two ways; their name and their belief system / popularity.

1. Name

To understand why Binks named the clan Guinea Pig Clan is important.
See, Binks's owner had not been a great owner: yelling at her, swatting her away, and never gave her compliments.
But her had a guinea pig whom he gave a lot of affection to.
So whenever Binks heard him saying stuff like 'good guinea pig' and 'you like that, don't you, guinea pig?' She assumed that he was speaking to her since she was always in the same room.

So, she has now assumed that her name is Guinea Pig instead of Ginks and has named the Clan GuineaPigClan as well as herself GuineaPigStar like LeafStar taught her to.

2. Belief System / popularity?

Now keep in mind that, by now you must understand that GuineapigStar isn't the smartest cat, so when she had heard from SpiderStar to 'worship the ancestors' she assumed that he meant literally bowing down towards their ancestors.
She made it a thing that you would worship the important StarClan spirits like leaders, deputies, med cats, etc. more than you would kits, apprentices, Warriors, and elders.

The elders one it controversial and due to this the next Leader, BuzzardStar, made it so that the elders were worshipped and respected much more than the others.

This made the cats want to become something new: they wanted to become important and popular, checking new things, trying to make large changes, establishing laws and getting territories in efforts to be worshipped more in the afterlife.

Sooo.... Here we are!
Forgot to mention, they later made it a rule that each queen has to pick one kit to survive in fear of overpopulation.