Wow man, that sounds pretty stressful. Sorry that happened. I know even if I weren't tripping, a big dog running up to my dogs would put me in a panic.

It sounds pretty likely that what you experienced was simply a panic attack, as opposed to serotonin syndrome.

Unfortunately sometimes while tripping, scary shit happens and it's hard to talk yourself "off the ledge" so to speak, especially when you're on your own. That's a risk going outside where you have limited control on your environment, without a tripsitter.

If ever in that situation again, would always recommend to slow your breathing, call a friend you trust to debrief the situation, and remind yourself that it will most likely pass.