If you look through your list of available emojis (the ones that you have), you'll see that the emojis have one of four different color outlines around them. It's either silver, gold, rainbow, or purple. Those denote from which type of the box the emoji is usually from (e.g., a silver outline denotes an emoji that is available from the silvers boxes). The rainbow box with the exclamation point denotes the rainbow box emojis, which are usually initially offered through an event as a reward. In addition to the ones RangerPooh mentioned, there is also Dug, King Triton, Magic Mirror, Rainbow Mickey, Edna Mode, Carpet, and Rainbow Unicorn. I don't know if any of the Group reward emojis are rainbows, but it would not surprise me.