If I play Rhasa directly from my trash (thanks to Endless Riches for instance), does it count itself for the energy cost reduction?

No — Rhasa does not count itself when played from your trash (via Endless Riches, Last Rites, or any similar effect). Step 1 of playing a card moves it from its zone onto the Chain (Rule 354); the discount is applied in step 3 (Rule 356), when Rhasa is already on the Chain and no longer in your trash — so only the cards remaining in the trash count toward its reduction.

Practically: with 7 cards in your trash, Rhasa costs 3 energy + 1 Chaos Power, not 2. FAQ #8430 covers exactly this sequence.