Weezing alternately shrinks and inflates its twin bodies to mix together toxic gases inside. The more the gases are mixed, the more powerful the toxins become. The Pokémon also becomes more putrid. Very rarely, a sudden mutation can result in two small Koffing twins becoming conjoined as a Weezing. If one of the twin Koffing inflates, the other one deflates. It constantly mixes its poisonous gases.
Pokédex entry #110
weezing in Pokémon GO
It's possible to hatch weezing from an egg?
No, weezing can't be hatched.
Which are weezing’s strengths and weaknesses?
weezing is a poison type Pokémon. poison type pokémons are strong against grass, bug, fairy pokémons but weak against poison, ground, rock, ghost pokémons. They are not affected by steel type pokémons.