So, I was browsing the internet the other day and read a question that perked my interest. The question was as follows, "What if all the spiders in the world turnt into tiny horses?"
This questions had me thinking, what WOULD happen if all the spiders in the world turnt into tiny horses? What kind of changes would this bring to the environment and how would the food chain change?
If you could answer these questions for me, I would be more than delighted. Thank you for your time!

What a fun question.  However, it's difficult to answer without knowing what 'tiny' means.  I mean, a horse 3ft tall is tiny - relative to a horse.

Given the biomass issues involved, lets assume that you're referring to all the spiders turning into horses of equal body mass (so very very tiny!):

Well, the simple answer is that this would be identical to if all spiders suddenly went extinct - spider sized horses would be unable to feed, or even to move around easily (as they can't climb up walls). Best case scenario is that those tiny horses that can get to a grassy area quickly, and start munching (though I'm not sure the energetics of converting cellulose heavy grass would work with a mammal stomach at this size).  There would be an explosion of insects (on average, a house spider eats 2000 insects a year), and subsequently other insect predators would boom, filling the niche left behind by the spiders/sad tiny dead horses.

If the horses were able to process grass, we may also see some kind of wierd effect on fields, with bare patches appearing as if the grass is diseased, as tiny horses begin to devour what they can.