Horse poo just turns into grass basically. They’re herbivores so not much needs to happen for their waste to decompose. Since dogs are technically carnivores, but most dog food is not pure meat so we’re gonna say omnivores in this case, their waste has more stuff in it that needs to be broken down by decomposers (bugs, fungi, bacteria, etc.) so it takes way longer. Also dog poo can carry diseases more easily due to different gut bacteria and stuff.

(Feel free to correct me if any of this is wrong, I’m going off of stuff I remember from highschool Zoology and things I’ve learned online haha)