Actually, both 1.0 and Beta 1.9 are correct.
Ender Dragon was first implemented in Minecraft version Beta 1.9 - which is Release 1.0.0. Yes, you read that right. Release 1.0.0 was initially named Beta 1.9.
At some point Mojang decided to change the version number to 1.0.0, though during beta phase the version number was Beta 1.9, so every pre-release version of 1.0.0 is tagged as Beta 1.9.
You can check more information on Ender Dragon's history section and Java Edition 1.0.0, and first ever version that has ender dragon Beta 1.9 Prerelease 4 (every link is linked to fandom).