I don't think it is 'most' but some do have non-standard animations
here chickens start in random idle state and switch idle animations a few times then walk right walk left and attack and back to random idle
attack: 1, 5, 6
idle0: 0, 1, 2, pause, 3, pause, 4, 1, pause, 2, pause, 3 pause, 4 pause, 1, pause, 2, pause, 3 pause, 4 pause (blink at start and every three cycles)
idle1: 0, 1, 2, 3, 4, 5, 6 (walk + headbob)
idle2: 1, 2, 3, 7, 8, 7, 8, 7, 8, 3, 4 (peck ground)
walk: 1, 2, 3, 4
Yes some are even more simple, maybe just 3 different speeds of the same frames...Even if you are not going to use unity3d.com it might be worth it to install to just check out example animations and timings... for example the pause at the top of the slime jump/walk cycle etc...
The animations and sprite sheets are just text files and can be deciphered/understood with some exploring.
Though you are free to arrange them any way you like.
Thanks.