Why was Popeye's bearded, burly foe named Bluto in some cartoons and Brutus in others?

Popeye was created in a newspaper strip for King Features Syndicate called Thimble Theatre, written and drawn by Elzie Segar.  The sailor eventually took over the strip and it was renamed in his honor.

When the folks at the Max Fleischer studio licensed the right to make cartoons of the sailor, they decided to use Bluto, a villain in a then-recent (1932) newspaper strip sequence, as Popeye's nemesis.  Segar had only intended Bluto to be a one-shot antagonist but the bearded brute wound up appearing in most of the cartoons produced by Fleischer and later by Paramount Studios.  Because he was such a part of the cartoons, he became an intrinsic part of the Popeye comic books and the newspaper strip, as done by those who took over after Segar's death.

When Paramount stopped making the cartoons, the animation rights reverted to King Features.  In the early sixties, King Features decided to produce their own low-budget Popeye cartoons for television.  The way the old contract had been structured, they had the rights to anything that had appeared initially in the newspaper strip but not to any characters created by the Fleischer or Paramount operations.

Because Bluto had not appeared much in the Segar strips, someone at King Features made the mistake of thinking Bluto had first appeared in the cartoons and therefore could not be used in their new films.  As a replacement, they designed a new, similar villain and called him Brutus.  Jackson Beck, who had been the main voice of Bluto for Fleischer/Paramount, was engaged to provide the same voice for Brutus.

So that's why Brutus was in the TV cartoons in lieu of Bluto.  In the newspaper strip and comic books, Bud Sagendorf — Segar's assistant and main successor — kept drawing the same villain he'd previously called Bluto but his name was not consistent..

Since then, they've largely reverted to the Bluto name…but every so often, he's called Brutus.  As for the character's look: There was one model for Brutus but Bluto went through several redesigns.  Nowadays on merchandise, any of these versions is likely to turn up and there are times when it appears the artist is trying to split the difference and do an amalgam.  So it's no wonder you're confused.