The marketability of a Thneed (situational irony)
A Thneed is portrayed as a ridiculous garment that the Lorax assumes no one will ever buy. Then, a consumer immediately snatches it up. It turns out there is a whole, thriving market for a somewhat silly product—the irony of consumerism.
The last Truffula Tree (situational irony)
In the Once-ler's pursuit of profit, he overuses and depletes the single resource he needs. This is ironic because the Once-ler's own activity ends up undermining its enabling conditions.
