Do animals really know when they’re about to die?

[Trigger warnings: aging, dying, death.]

Do animals really know when they’re about to die? 

I don’t want to go into the scary details here, but the physical and mental decline that I have been experiencing over the past year feel more than just the “natural decline that starts in the fourth decade of life”. I’m having really acute, scary episodes of both mental and physical decline that feel beyond “normal”, beyond “natural”, i.e. actually unhealthy, heralding the beginning of the end. This doesn’t feel like “natural aging” or like “having reached the peak and now slowly going downhill”: this feels like I’ve suddenly turned a sharp corner and come to stand on the edge of a precipice. 

I’m going to reach out to my doctor and get all the tests I can (I know from family history one of the possible causes of this, unfortunately). But if nothing shows up, can I still trust the “animal instinct of the approaching end”? And if so, what do I do?