The reason Heath is identified only by his first name while the other characters are listed by first and last is because Heath's last name was an open question in the early episodes.
This is a prime example of why many long-running shows today update their opening credits each season. It made perfect sense in the first season of the '60s western drama "The Big Valley" to only list Heath (played by pre-"The Six Million Dollar Man" Lee Majors) by his first name because his questionable status as a Barkley was central to the season's overarching plot.
He arrived at the Barkley family ranch in the first episode, claiming to be the illegitimate son of the late Barkley patriarch. Naming him Heath Barkley at the beginning of the show would have given away the fact that the family eventually, after several episodes, accepted him as one of their own.
The problem you point out is that, while Heath's familial status became less controversial over the subsequent 111 episodes, the credits didn't change.
Majors was actually at the center of a similar confusing-credits situation just a few years later.
He appeared in a season-long arc as Roy Tate on Season 9 of "The Virginian" in 1970 and 1971, and while his character wasn't actually introduced until the fifth episode, he appeared in the credits from the very beginning of the season, prompting fans to wonder who this "Tate" guy was. (The show seemed to play into this a little — the title of the fifth episode was "The Mysterious Mr. Tate.")
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