Here is how the S.S. Headliner Crew met Carnival Cat and discovered the last Six New Elements of Harmony in The Tale of The SquarePants Master Ship Builders.
The next day.
SpongeBob SquarePants: This is it, Guys. Only Six more Elements of Harmony.
Patrick Star: Let's hope nothing crazy can go wrong.
Carnival Cat: Hello. Everypony!
Mr. Krabs: Who the Barnacles is that!
Titanic: That's Carnival Cat!
Olympic: The most insane enchanted pony of all in Equestria!
Britannic: She eventually became friends with Twilight Sparkle, Her friends and the Cutie Mark Crusaders.
Titanic II: What're you doing here, Carnival Cat?
Carnival Cat: I'm here to help. (brought out the last six elements of harmony) Look what I have.
Trixie: (gasps in amazement) The Last Six of the New Elements of Harmony! (then, she saw the elements glowing in front of her, the CMC, Discord and Carnival Cat) What in Equestria's name?
Apple Bloom: What is happening to us?
Scootaloo: I don't know!
Sweetie Belle: I think new elements have chose us!
Discord: You could be right, Sweetie Belle!
Carnival Cat: Well, I'll be darn!
Rarity: (amazed) We're so happy for you and your friends, Sweetie Belle!
Applejack: It's good thing you're with us, Little sis!
Rainbow Dash: You go, Squirts!
Twilight Sparkle: This is it! We got all the new Elements of Harmony!
SpongeBob SquarePants: And the Element of Faith, Showmanship, Bravery, Patience, Enthusiasm and Determination has chosen Carnival Cat, Trixie, Discord, Apple Bloom, Sweetie Belle and Scootaloo!
Oswald the Lucky Rabbit: That'll show Merlock what not to mess with!
Mr. Krabs: Let's go!
Gremlin Gus: Let's go back to Equestria!