LINE Friends BT21 Sticker is a pilot project from LINE Friends in collaborations with K-pop sensation BTS.
Basic Guide: How to Make Animated Stickers
A basic guide on how to create animated sticker for Apple iMessage and LINE.
PNG Animator, The Best APNG assembler for Mac
PNG Animator app probably the best APNG assembler for Mac. Friendly user interface and very easy to use, has preview, compression option to optimize the output.