Well… It depends.

Was it truly deliberate, as in “I want to poison this dog, so that it dies”? If yes, then there is hard to forgive, maybe not possible. The only way would be if they reported themself to the police and offered a sincere apology, and then I would not let them near my dog as long as it lives. But could maybe forgive in time if they did all that.

Deliberate is also not black and white. Were they negligent? As in knew that dogs can’t really have onions, but did fed them food with just a little bit of onion in it, thinking it would be fine? Negligence can be forgiven, but only with a sincere apology.

I would forgive them if it was not deliberate at all. Even with a pretty bad apology.

If this person is important in your life, or are closely related, I would explain to them that the apology did not feel sincere, and that you consider it hard to forgive. Ask that they try again.

Also, do forgive them for your own sake, no reason to be angry in the long term, but you don’t have to trust them.

If it was a child that did this everything is very different. The answer assumed a person 18-20+

That’s my take.