Not comprehensive, but some of the taco fillings I saw in a recent taco crawl. Good link to Mexican food terms at the end.
There are some odd balls in the list like Jamon Cocinado and Chile Verde.
Even though I worked in Mexico City for a year ... I was WORKING. When I ate, I focused on tortas. So, going into my crawl I was clueless about many of these terms. I heard them, I just wasn't sure what they were EXACTLY.
So, for anyone as clueless as me, here they are. I hope those more in the know will let me know if I have any of this wrong.
AL CARBÓN - charbroiled meats.
AL PASTOR marinated pork cooked on vertical spits like Mid-Eastern shawerma, although it is just usually marinated pork that is fried, baked or roasted.
AL VAPOR - steamed meat, often from a cow or goat's head
BARBACOA slow-cooked shredded beef or lamb. It is usually meat from the head. Originally the head was placed in a pit lined with coals. Now it is usually steamed
BIRRIA slow cooked meat that is stew-like
BIRRIA DE CHIVO goat
BIRRIA DE RES beef
BUCHE - pig's stomach
CABAZA beef head
CARNE ASADA grilled beef
CARNE MOLIDA ground beef
CARNITAS Pieces of pork slow-cooked, then fried;
CHICARONNES fried pork skins
CHILI COLORADO - red chili sauce often with beef
CHILE VERDE green chili sauce meat can be anything but usually chicken
CHORIZO crumbly Mexican pork sausage
DESEBRADA shredded beef
JAMON COCINADO cooked ham / pork
LENGUA tongue, or as one menu listed, bovine tongue
POLLO ASADO grilled chicken
SESOS beef brains
SUADERO rose meat Probably beef. See this Chowhound link for some ideas
TRIPITAS fried tripe (usually the lining of a cows stomach. Can be pig or sheep)
Cebollitas - Green onions or scallions
zanahorias en escabeche marinated sliced carrots usually with onions, and oregano
The following link, while not comprehensive, has some other Mexican food terms and background. There is a nice list of antojitos.