That looks like a very typical Dahlia clump. Did you actually see them growing there or was it perhaps in a pile of last years garden waste someone dumped there. Those tubers usually grow deeper in the soil.

There are recipes for cake using Dahlia clumps, so I reckon they can't be too bad.

Probably,
1) the dog ate something else earlier and showed delayed reaction when picking up that bulb
2) more unlikely there is stuff in Dahlias which is toxic for dogs.
( never ever heard of an Dahlia poisening )

Good luck,
Lin

I agree with Linaria on all points.

It's possible the tuber could have been dusted with something toxic to dogs to help it through winter storage.