Which group pioneered this K-pop song characteristic ?

Hi, I was a gen 2 K-pop listener and stopped after that.

Was curious who pioneered this distinct K-pop song characteristic (I hope i describe it correctly), where the song would transition into a different beat and tune so that it seems like there’s 2-3 songs mashed into the one song ?

I remember when I first got into K-pop and listened to DBSK/TVXQ’s Triangle. I thought it was so odd, and definitely felt like there were a few songs mashed into one. Then shortly after that, I saw this characteristic pop up again in big K-pop hits from SM and YG.

What was the first K-pop song that introduced this?