Verified answer

To find the new mobs, you must activate hardmode. The hallow biome, new mobs, new weapons, and anything else is only in hardmode. To activate hardmode you must defeat the wall of flesh. To summon the wall of flesh drop a voodoo doll into a lava pool in hell. Look for a demon holding a voodoo doll in its feet and kill it. Be warned, hardmode is extremely difficult and should only be activated when you have nearly the best items in normal mode. The corruption and hallow start spreading at a fairly rapid pace and you need to be able to contain them. Once you are in hardmode, you cannot change it back unless you use a third-party program. Check out the link below for more details.