Monsters that are rare and up can evolve. To simply evolve a monster, the only requirements are that the monster and feeder are level 25-40, depending on rarity. Specials do not even need to be leveled at all. If an evolution were to be done with the specials and feeder monster at the bare minimum level, while much less work, this would result in the most imperfect evolution possible.
Rare monsters and event monsters only require 4 specials of the same element for an evolution. Reward and ultimate grab monsters super+ require a candy of the monster's rarity in addition to 3 specials of the same element.
You can not reverse or undo evolutions. The base attack of your monster's stats will be permanently lower. Things like passive abilities and boosters go off of base attack, so their efficiency will be reduced. The monster itself will also be worth much less for trading, if anything at all.
Second evolutions can only be done with supers or better and will make your team much stronger. However, a second evolution is not easy. Firstly, you need your evolved monster that you want to sevo and also a feeder that has been evolved as well. Rather than regular specials, a second evolution requires crystals.
Crystals are harder to obtain than regular specials, but there are multiple ways to get them. Lost Temple is the most popular way. When a crystal is defeated, it has a chance to drop 1-3 crystals, depending on what number crystal your troop is on. During raid events, extreme bosses have a chance to drop a crystal as well. Benchmark rewards often have crystals when you achieve moderate amounts of trophies. Crystal Siege typically has crystals as rewards for goals being met, such as winning a match or certain number of matches. The trader in the troop hall typically has some options to trade for crystals as well.
The most difficult crystals to obtain are the required crystal of your monster's rarity. The chance for these crystals to drop is much lower than the elemental crystals.