—Everyone, everywhere., Friendship is Magic, Part 1
Sea Ponies appeared in the two G1 specials and the G1 MLP movie. Inhabitants of the lakes and seas, they love water polo and spontaneous song and dance numbers. For some reason they are very fondly remembered by fans, mostly due to their tremendously odd designs and THAT SONG!
THAT SONG!"Call Upon The Sea Ponies" is the song sung when the sea ponies first appear, and introduced their famous "Shoo-be-doo" catchphrase in the chorus. There were a lot of (mostly forgettable) songs in G1, but this became famous for being horrendously catchy and actually quite good. Though cheesy, the lyrics are perfectly servicable and the singing and harmonies are pretty good, especially given the terrible sound quality of most clips.
As noted, the song is horrendously catchy. Be warned against clicking this link, as some have been found still humming it weeks later.
Place In Friendship Is Magic FandomAccording to Lauren Faust there was a script for FiM that included the sea ponies, but that it never got past the draft stages. As such they currently do not exist in FiM canon.
That doesn't stop the fans however. Fan art of them in G4 is very popular, and they have also appeared in many fanfics. Sea Pony Lyra in particular is a full fan character who's crossed over with herself several times.
They've also had musical tributes. When prolific musician Eurobeat Brony ran out of FiM tunes to adapt, he adapted instead "Call Upon The Sea Ponies"