A good idea, while you cannot upgrade their attack, you can get a pack and upgrade their health and speed. They become useful as attack dogs this way. Another usage is to transport wood early game. Hope this helps!
These guys are incredibly useful from being a portable light that works for a long time to being a cute little friend in hardship. Do not kill these, they are too cute, I died to a basilisk on a mana and the bulbdog jumped off and ran strait for it... I now have a graveyard near my base. R.i.p Bulbdog dude