Generic Method Parameters in Golang

You can't do precisely what you ask for in Go. Closest way to do things in Go would be something like this as shown in code below:

type Ids interface{
  Id() int
}

func (this Mammal) Id() int{
  return this.ID
} 

func (this Human) Id() int{
  return this.ID
} 


func Count(ms []Ids) *[]string {
...
    IDs[i] = strconv.Itoa(int(m.Id()))
...
}

func main(){
  mammals := []Ids{
    Mammal{1, "Carnivorious"},
    Mammal{2, "Ominivorious"},
  }

  humans := []Ids{
    Human{ID:1, Name: "Peter", HairColor: "Black"},
    Human{ID:2, Name: "Paul", HairColor: "Red"},
  }
  ...
}  

here is the working example

Use interfaces instead of concrete types, and use embedded interfaces so the common methods do not have to be listed in both types:

type Mammal interface {
    GetID() int
    GetName() string
}

type Human interface {
    Mammal

    GetHairColor() string
}

And here is the implementation of these interfaces based on your code which uses embedded types (structs):

type MammalImpl struct {
    ID   int
    Name string
}

func (m MammalImpl) GetID() int {
    return m.ID
}

func (m MammalImpl) GetName() string {
    return m.Name
}

type HumanImpl struct {
    MammalImpl
    HairColor string
}

func (h HumanImpl) GetHairColor() string {
    return h.HairColor
}

But then of course in your Count() function you can only refer to the method and not the field of the implementation:

IDs[i] = strconv.Itoa(m.GetID())  // Access ID via the method: GetID()

And creating your slices of Mammals and Humans:

mammals := []Mammal{
    MammalImpl{1, "Carnivorious"},
    MammalImpl{2, "Ominivorious"},
}

humans := []Mammal{
    HumanImpl{MammalImpl: MammalImpl{ID: 1, Name: "Peter"}, HairColor: "Black"},
    HumanImpl{MammalImpl: MammalImpl{ID: 2, Name: "Paul"}, HairColor: "Red"},
}

Here is the complete working code on Go Playground.