Reopening this issue (MC-307946) as zombie horses are now capable of spawning naturally on their own, removing the “mayhem” part. This is not consistent with camel husks and skeleton horses, nor with bedrock as they cannot spawn on peaceful too.
Making them spawn on peaceful breaks their mechanic.
Thank you for your report!
We're tracking this issue in MC-307946, so this ticket is being resolved and linked as a duplicate.
That ticket has already been resolved as working as intended, which means this is not considered a bug and won't be fixed. Please do not leave a comment on the linked ticket.
If you haven't already, you might like to make use of the search feature to see if the issue has already been mentioned.
Quick Links:
📓 Bug Tracker Guidelines – 💬 Community Support – 📧 Mojang Support (Technical Issues) – 📧 Microsoft Support (Account Issues)
📓 Project Summary – ✍️ Feedback and Suggestions – 📖 Game Wiki