Can a Chihuahua eat mushrooms?
Store-bought, yes. Wild mushrooms, NO.
How much: 1 small cooked mushroom, with no salt or oil. Avoid raw.
Why
White button, portobello and shiitake mushrooms from the grocery store are safe cooked. Wild mushrooms can be deadly: if in doubt, don't give it.
Mushrooms in a Chihuahua's diet
Keep in mind how tiny the portions are for this breed: an adult Chihuahua eats roughly 1 to 2.3 ounces (25-65 g) of food a day depending on his weight, and treats shouldn't go over 10% of his daily calories. That's only a small fraction of an ounce. What looks like "a little bite" to us can be half a meal for him.
This information is for general guidance and is not a substitute for advice from a licensed veterinarian. Every Chihuahua can react differently; if in doubt, always check with a professional.