Seems Typescript is not smart enough to understand the type when looping it, so I need to simplify it when I want to process the array, and let the complex type only for type cohertion:

type Item<Id extends string> = {
  id: Id,
  isFixed?: boolean
}

type FixedItem<Id extends string> = Item<Id> & {
  isFixed: true
}

type NotFixedItem<Id extends string> = Item<Id> & {
  isFixed?: false
}

type Items<Id extends string> = NotFixedItem<Id>[] | [FixedItem<Id>, ...NotFixedItem<Id>[]]

const items: Items<'dog' | 'cat' | 'horse'> = [
  {id: 'dog', isFixed: true},
  {id: 'horse', isFixed: false},
  {id: 'cat'},
]

// items here is a much more general type 
const loopItems = <Id extends string>(items: Item<Id>[]) => items.map((item) => {
  // item is understood as Item<Id>
}

// the complex type Items<Id> is an specific case of Item<Id>[] so the param is valid
loopItems(items)