Animals that are scavengers and carnivores like vultures and eagles will have the strongest stomach acid because they have to break down things like meat, bones, cartilage, and skin. For example, the common buzzard’s stomach acid has a pH of 1.1! The animals with the weakest stomach acid will be animals that eat plants like simple grains. This includes hamsters, deer, monkeys, and camels. To answer your question, the animal with the weakest stomach acid is the llama with a pH of 7.3.

I would expect that it would be some kind of aquatic animal whose gut is open to the water outside of the animal. Placozoans (basically just microscopic blobs of cells tied together by proteins) form temporary guts underneath them when they find food, and it's possible for water to flow in-between the cells into this temporary gut. I would not expect the internal fluid to be that acidic.

Here's the best answer I found - it's in a scientific publication: article

Before commenting on the reading of this article, yo have to know that the unit to measure acidity is pH, which stands for potential of Hydrogen. A pH value is a number from 1 to 14. 7 as the middle (neutral) point; alues below 7 indicate acidity which increases as the number decreases, 1 being the most acidic.

Going back to the article, the answers are confusing, because one type of monkey has a weak stomach acid [pH 6], and another type of monkey has a stronger stomach acid [pH 2]. Llamas have a stomach pH of 7, which isn't an acid at all. Camels' stomachs have pH 6. Echidna's and sloths have stomach pH's of 7. All the other animals and birds in the list have stomachs with acids that are at least a little weaker than these.

The article says there isn't much data about the strength of stomach acids so the numbers might change if there were more data - if the animals' stomachs were tested again.

When I searched for your exact question, "What animal has the weakest stomach acid?" I got answers from less scientific sites. ScienceLine has an interesting answer about stomach acid and digestion:

Can you tell me the animals with the strongest stomach acid?