is there somewhere a prizelist of items you can get at the Mallarchy Pond?

The prize for Mallarchy Pond are the Carnival Ticket(s).

Some time back staff added random Trick or Treat Morostide Items to the carnival prize pool as a bonus. This was supposed to be a temporary thing, but it still hasn't been removed. I don't know if all old Morostide items are included, or just specific years. You can see all the old Morostide Trick or Treat Item here.

Note that you can win these items through all the carnival games, not just Mallarchy Pond.