Everytime Ultra Beasts get re-added to the raid rotation, this subreddit gets an influx of users asking, "What is this?" To (attempt to) mitigate seeing this question asked multiple times a day for the next two weeks--it's an Ultra Wormhole.
Ultra Beasts come from Ultra Wormholes.
Ultra Beasts are (simply put) a group of extradimensional Pokemon.
Why aren't they in the usual 5 star raid eggs. For aesthetics? It's in line with the main line games, I guess. Technically Ultra Beasts are NOT legendary Pokémon, so they don't get the 5 star egg typically used to denote a Legendary Pokemon appearing in that raid.
EDIT: We regularly get new players joining the app and this sub all the time. And people who play the game are going to range from no knowledge to extensive knowledge about Pokemon lore. We understand that new people to the Pokemon franchise are going to have questions, but either the Mods need to add an FAQ or a weekly question thread, or people are gonna have to scroll a bit to see if someone else just asked the same question.