Lucy Tomas

Very simple plugin that will search for all the first images of your published posts, pages or custom post types and set them as the featured image.
<ul>
<li>You can choose the post type you want to run the plugin on.</li>
<li>Option to overwrite the already selected futured images.</li>
<li>Option to select From / To date.</li>
</ul>

Set All First Images As Featured

still works

It simply didn't recognize any images in any of the posts, and so didn't work. :(

Didn't see the images in the posts

IT works on 5.0.3

Sets the first image of your posts, pages or custom post types as the featured image.

xaraartech

Abuzer

Adds functionality to search images from post page and attach as featured image. 
You will require to create an API key on bing for search API. 
GOTO: https://azure.microsoft.com/en-us/services/cognitive-services/bing-image-search-api/ or https://dev.cognitive.microsoft.com/docs/services/56b43f0ccf5ff8098cef3808/operations/571fab09dbe2d933e891028f 
And generate your “Ocp-Apim-Subscription-Key”

External featured image from bing

Adds functionality to search images from post page and attach as featured image.

Set All First Images As Featured vs External featured image from bing