ANIMATED is the lump created by TeamTNT for Boom which defines Doom-style animations sequences. Its format is the same as the one originally used internally by Vanilla Doom. It's a binary lump, unsuitable for hand editing, but you can use either the TeamTNT-provided swantbls tool or modern lump editors such as SLADE to customize it.
Only use the ANIMATED lump if you want your mod to run on Boom or PrBoom+. Unlike EDF and ANIMDEFS, but similar to PNAMES and TEXTURE1, it must contains everything from port's original data. Open eternity.pke from the base/doom folder and copy the ANIMATED lump from there into your own PWAD. Use one of the editing tools mentioned here to update the lump.
SMMU-style swirly animations can be set in ANIMATED if you set duration per animation frame to 65536. You cannot create swirly multi-frame animations however.