I believe both the Bush and Obama administrations indirectly contributed to the conditions that allowed ISIS to rise. The Bush administration's invasion of Iraq and the destabilization that followed created fertile ground for extremist groups, while the Obama administration's handling of the Syrian conflict and limited intervention contributed to the chaos ISIS exploited.
As for Trump, he's using his typical rhetoric, where he blames Democrats for any negative outcome. I don’t think he genuinely believes Obama literally started ISIS, but he likely believes Obama's handling of the Middle East contributed to the conditions that allowed ISIS to emerge. While that’s only part of the story, since factors from both the Bush and Obama administrations played a role, Trump simplifies it to blame the Democrats. Do you have any evidence that shows he doesn’t think it was solely the Democrats’ fault? Even if you have proof, isn’t it still fair to call the argument that Obama is solely responsible for ISIS complete bullshit?