There's an option to do that in Mandelbulb 3d. When you're in the animation window, there's a button called "Render Preview". I just activated that at down scaling of 1, render every n'th frame at 1 and unchecked the "Fast+inaccurate" option.



Once it's done rendering the preview, I can save it as a gif.



Though you should do it once at a low setting (render every 2 or 3rd frame at a downscaling of 3 or 4, just to get it down to a thumbnail preview) so you can save that animation as a thumbnail for your animations.