It's getting popular due to meeting you when you go to school with bread in it! Tickle girls heartily as well as lots of lost memories soon.
Their high knockback count and low health makes them more deadly, as a few hits from meatshields like Mohawk Cat and Manic Mohawk Cat easily knock them back, and take note that they don't have an actual attack cooldown, they only have delayed attacks due to their attack animation. Thus, the more they get knocked back, the more damage they cause to your defenses.
Avoid using the normal Cat Cannon against Brollows, as it will cause them to all attack again (think of Awakened Bahamut or Yukimura being hit after they land a blow). High numbers of hits per second is very bad against them, but powerful single blows from spammable, bulky and heavy-hitting cats, like Jamiera Cat, Island Cat, and Crazed Whale work well against small packs of Brollows. However, for large packs of Brollows (for example, the stage I'm onto you), more expensive AOE cats will be required, such as Ururun Cat.
Incidentally, D'artanyan's unevolved form's blindspot does not include Brollows, who will rest at the 150 mark instead of 140. This means that, in theory, a D'artanyan can obliterate Brollows, though it is better to have meatshields take hits instead of D'artanyan.
Kotatsu Cat, Kai or Sodom can help a lot as walls to gather many Brollows in the same area to kill them all at once.
This is the 2nd fastest enemy in the game. The only unit faster than him is Assassin Bear.
The design of Brollow is based off of a swallow, a type of bird well known for its black coat and v-shaped tail. Swallows are also a symbol of good luck, fidelity in marriage, and fertility in Japanese culture.
Brollow closely resembles a gannet, known to be the "missile bird."
Brollow is also based off of the common swift, the world's fastest bird in horizontal flight, which can be clearly seen as Brollow moves forward. It also has a shorter, smaller v-shaped tail, though it is still noticeable. Another similarity between Brollow and the swift: the wings are slightly more forward on the body than they are on a swallow.