As much as people like to delve into conspiratorial gossiping, making swooping statements about how Google and Amazon work together, there's often much simpler and more reasonable explanations.

For one, you are one of billions of people browsing both these sites today, it's bound to happen to some of you.

But what prompted you to look up the horses stuff? Sometimes it's an article, a social media post (reddit and Lemmy count), a radio segment, etc. That often leads a group of people to look up the same stuff en-masse.

Its also possible that you've visited other sites about horses that have put you in that cohort, where manufacturers have placed their own tracking pixels whose info they can supply to Amazon for targeting.

The reason why a Google / Amazon collaboration seems so unlikely is that they are competing. Not just at large, but in this specific case. Those recommendations you see on Amazon are ads too. People pay for them, and use specific targeting rules to find people to click on them. This is what both Amazon and Google sell, access to specific eyeballs (eg. males in their late 30's who have once shown interest in motorsports). This is their secret sauce. They'd be crazy to allow that information to flow to a competitor with their own ad platform.

I know this goes against the grain here, so feel free to downvote, but keep in mind that conjecture and wildly inaccurate gossip about what these giant companies do often muddies the waters and makes it much harder to attack them on the shady and downright evil stuff that they do do.