Hi guys,
This is what makes Valve's games so awesome - a superior level of details:
Houndeye behaviours:
https://www.youtube.com/watch?v=9jO-P3kXlCI
https://www.youtube.com/watch?v=Z8_R2OMgNUY
Seagull pooping at your car:
https://www.youtube.com/watch?v=knSBrO3GBXY
Vortigaunt raising deads:
https://www.youtube.com/watch?v=RV9ooD5rx1o
I don't know how much time Valve spent on modelling those :-)
But that's exactly what makes their games fun!
Edit:
There are more details about implementation of AI:
https://developer.valvesoftware.com/wiki/Category:AI
https://developer.valvesoftware.com/wiki/AI_Learning:_CombineSoldier
http://aigamedev.com/open/article/halflife-sdk/
Have a nice weekend,
Les