Complex sentence: It was believed among the Lakota that if you had a dream or vision of birds you were destined to be a medicine man, but if you had a vision of the Wakinyan Thunderbird, it was your destiny to become a heyoka, or sacred clown.
Split into simpler sentences: It was believed among the Lakota and that if you had a dream, or vision of birds. You were destined to be a medicine man; but if you had a vision of the Wakinyan Thunderbird, it was your destiny to become a heyoka, or sacred clown.