Step one : take a large dog from the BBB
Step two : implant a 2-dose auto-injector from Augmentation, filled with K-10 and Immortal Flower (from Arsenal)
Step three : hillarity ensues
Voila, instant killing machine for under 3000

.
Works on metahumans, too.
My favourite is the killer gnome.
If you want a cybered critter without building a biodrone, just use the normal critter stats from the BBB and combine them with cyber- and bioware you can find there and in Augmentation.
The critter list is quite limited, though.
We're all still waiting for
Running Wild, the critter sourcebook in the making since SR3.
It's a bit like the SR version of
Chinese Democracy, but without arrested bloggers.