dynamically create forms using a STRING-Collection of common programming errors

Declare the NewForm variable as TForm:

var
  NewForm: TForm;
begin
  NewForm := TMyForm.Create(Tab1); //compiles OK
  NewForm := TMyOtherForm.Create(Tab2); //also compiles OK
end;

I’m assuming TMyForm and TMyOtherForm both are derivatives of TForm.

DRY

You can also reduce your repeating code using a class reference variable, like this:

procedure TForm1.ShowFormOnTab(pProcName:String);
var
  NewForm: TForm;
  ClassToUse: TFormClass;
  NewTab: TTabSheet;
  FormName: String;

begin
  NewTab := TTabSheet.Create(PageControl1);
  NewTab.PageControl:= PageControl1;
  NewTab.Caption:='hi';
  PageControl1.ActivePage :=  NewTab;

  if pProcName='ProcfrmSetupItemCategories' then
    ClassToUse := TfrmSetupItemCategories
  else if pProcName='ProcfrmZones' then
    ClassToUse := TfrmZones
  else
    ClassToUse := nil;
  if Assigned(ClassToUse) then
  begin
    NewForm := ClassTouse.Create(NewTab);
    NewTab.Caption := NewForm.Caption;
    //if you access custom properties or methods, this is the way:
    if NewForm is TfrmZones then
      TfrmZones(NewForm).ZoneInfo := 'MyInfo';
  end;
end;

Register your classes and then create the forms from a string

As Sir Rufo points in his comment, you can even go further registering your classes (I’m not sure if this can be done in Lazarus, that exercise is up to you).

First, register the form classes you want to instantiate from the class name, previous to any call to your ShowFormOnTab method, for example:

procedure TMainForm.FormCreate(Sender: TObject);
begin
  RegisterClass(TfrmSetupItemCategories);
  RegisterClass(TfrmZones);
  //and other classes
end;

Then, you can change the code to get the class reference from the class name string:

procedure TForm1.ShowFormOnTab(pProcName:String);
var
  NewForm: TForm;
  ClassToUse: TFormClass;
  ClassNameToUse: string;
  NewTab: TTabSheet;
  FormName: String;

begin
  NewTab := TTabSheet.Create(PageControl1);
  NewTab.PageControl:= PageControl1;
  NewTab.Caption:='hi';
  PageControl1.ActivePage :=  NewTab;
  //get rid of 'Proc' and add the T
  //or even better, pass directly the class name
  ClassNameToUse := 'T' + Copy(pProcName, 5, MaxInt);
  ClassToUse := TFormClass(FindClass(ClassNameToUse));

  if Assigned(ClassToUse) then
  begin
    NewForm := ClassTouse.Create(NewTab);
    NewTab.Caption := NewForm.Caption;
    //if you access custom properties or methods, this is the way:
    if NewForm is TfrmZones then
      TfrmZones(NewForm).ZoneInfo := 'MyInfo';
  end;
end;

That way, the code remains the same for any number of classes.

For more info about this, take a look at Creating a Delphi form from a string in delphi.about.com.