I used it on my pet Vahva and this is what I got:

Vahva gained: 0 level(s) 0 strength 0 defense 0 intelligence 0 health points 0 speed points

Is this a bug?

No... I think it gives 0-1 intellect, and I'm pretty sure all boosters give you a run down of all stats even if they only have a chance to affect one of them. You got unlucky and got 0 ):

UGHHHH I wish I would've known, I would've sold it instead of used it on my pet.

Well.... there is a reason why it's cheaper than intellect boosters and guarantee at least one point.

k then