Forum:Stratigie or cheat way to win?

I lost cause of a certain stratigie that seems kinda cheating, but i don't know, I just started dueling not to long ago.

I had Fairy Dragon on the field against Tree Otter, then my opponite summoned Hyena and then used Creature swap to take my Fairy dragon for his Hyena. then he destroyed the Hyena I got and said, because it went to his graveyard he can summon 2 more Hyenas to his field and attacked me with both. Is this legal? If it isn't can you please explain?