Let me explain a little about animations on the md2 and md3-file and how Sauerbraten uses them;
Animations exist from a certain amount of frames. As they are arranged just as a bundle of frames (like frame 0, frame 1, frame 2, etc), they only carry a number to call them up within Sauerbraten and be rendered on your screen. This means that a model has (for example) 102 frames, which is being divided into animations that can be called up within Sauerbraten (for example,
md2anim death and
md2anim swim). However, the division has the con that you need to call up a certain amount of frames of the model (like frame number 97 to 102 are showing of a swimming model). So you need to call up those frames by specifying the frame it starts (counting within Sauerbraten starts at 0!) and how much frames the animation contains.
As luxury, there is an option as well to specify at which speed (in frames per second) the animation should run (so you can use a low framerate while playing it on a longer amount of time) and on top of the bill, a slot to announce towards Cube Engine 2 which priority the animation has against other animations (for example, a jump animation shouldn't override the running animation else it would continuous jumping).
So the end result is:
"number of starting frame"
"amount of frames"
"speed of frames being displayed"
"priority against other animations"
md2anim forward 1 5 13 1
This means the animation is meant for displaying forward of the model, starts at the second frame, has 5 frames, should be rendered at 13 frames per second and should override other animations after cycle.
Now about your question of the animations existing in monsters which appear currently in Sauerbraten;
Why? Because those models once were made for Quake 2, which did need all such animations as crouch, salute, point, wave, etc (I'm not sure, but I think Quake 2 required animations which had a separate set of frames to be called up by some name, instead of just specifying a frame number).
animoverride reveals, there are currently 37 different tags to call as animation. Those 37 different tags aren't all for playermodels (only 29 of them are), Mapmodels (called "mapmodel"), triggers (called "trigger") quad (called "powerup"), armors (called "shield") hudguns (called "gun idle" and "gun shoot"), vwep (called "vwep shoot" and "vwep idle") as well.
No, you can't add a custom animation-tag without changing the source code.