Trinkets are items that may be attached to horses for decorative purposes. They may be found in Seasonal Gift Boxes and through events, and were previously available in the trinket shop or in Prize Boxes and Daycare Gift Boxes. There is no practical purpose to attaching trinkets to horses, with the exceptions of the Tidal Serum and Water Orb. However, some trinkets have additional uses outside of being given to horses.
Collections are groups of related trinkets that were all obtained during the same site event. Placing a trinket into its collection is permanent, and removes one of that trinket from the player's inventory. Some collections give a reward upon completion, while others do not.
A complete list of trinkets can be found here.