How do I grow my inventory?

Someone told me I could get super cool, legendary, fly ride pets if I played for like 2 months. Thing is, ive been playing for almost 3 months and the best I got is a golden unicorn that everyone lowballs for. How do I grow my inventory without spending real money? how in the world are you guys so rich? This is really frustrating me and now with the new update, im not trading my megas or neons