If I leave my mash tun unattended for a few days after brewing it smells like a dead animal covered in vomit. What is the name of the bacteria/fungus/other that excretes this smell?
The answer is: Every bacteria that exists in your local area. Lacto, brett, wild yeast, and less pleasant wee beasties. Its unlikely that any bacteria on your grain survived the mashing process. Not impossible, but unlikely.
I have no doubt that its a combination of all of those factors.
My advice would be to never let anything sit around dirty. Clean equipment is happy equipment.
That smell is mostly form pedicoccus. Its a bacteria that work aerobically and it has a vomit like smell. Lacto is anaerobic and has a fairly clean aroma.
When doing sourmashes (leaving the mash for a few days at ~110-120F) there will often be a layer of nasty smelling malt on the top that can be scooped out. The mash underneath is soured and very "clean" tasting aside from the sour lactic acid taste.
I always thought vomit smells in brewing come from butyric acid and butanoate, produced by Clostridium species. Wikipedia agrees: http://en.wikipedia.org/wiki/Butyric_acid
Butyric acid boils at 164 C, so boiling your wort won't get rid of it, but it's worth a shot. I know Papazian and Brew Your Own recommend one long sour mash at 110F, but why aren't we making our sour mashes like sourdough bakers make starters? Here's what I do to get a clean lactic sourness:
Combine ~100 mL of boiled wort with a handful of pilsner malt straight out of the bag. Give it a good shake to provide oxygen for yeast growth. After 12 hours, dilute 1:2 into more boiled wort, add half a handful of malt, and shake. Repeat until you see bubbles, then repeat a few more times. Voila, you've got an alcoholic and sour culture. The lactic acid bacteria/yeast symbiosis has grown much faster than any clostridium, enteric bacteria, or molds, and has come to dominate, even preventing clostridium growth -- but only because of the frequent dilution with fresh wort.
Lactobacillus is a facultative anaerobe, which means it can tolerate oxygen, but the yeast depends on oxygen for growth, so don't forget to shake (remember sourdough is not anaerobic).