Why does this plum/apricot hybrid look like brains?
It is difficult to say, but it is likely due to disease. Many plant diseases have the effect of convincing plant tissues that they are some other organ than what they actually are, which leads to deformations.

I was not able to find anything that looked as dramatic as the image that you show, but there are similar diseases in stone fruits that deform fruit:


Plum pox potyvirus: http://download.ceris.purdue.edu/file/1424
Plum pocket fungus/"Bent banana disease": https://pnwhandbooks.org/plantdisease/host-disease/prune-plum-prunus-spp-plum-pockets, http://shropshireprunedamson.com/a-bad-case-of-bent-banana-disease/


I hope that this is helpful.