Here are several suggestions:
1. In pascal it is common that most private variables have a letter F as prefix. So instead of Health you would us FHealth. This way you can have verry similar names for private variables and prperties. This increases your codea readabiliy a lot.
2. Replace mytargetID varaible with pointer to monster class. This way you allow yourself to easily get certain information from that monster (Position, Health, MosterType, etc.) by simply accesing property of that class (no need to search through a list of all monsters to find which has apropriate ID).
3. In OnHit procedure you are substracting armor value from attack (damage) value and then checking to see if that is larger than 0 while you could easily just check to see if armor value is greater than attack value.
Or even better simpy substract armor value from attack value and if that is largen than 0 reduce the health by that result.
Code:
procedure OnHit(attack: Byte);
var Damage: Integer;
begin
    damage := attack - armor; //Reduce damage by armor
    armor := armor - attack; //Reduce armor durability
    if armor < 0 then armor := 0; //to avoiud armor being negative mumber
    if damage> 0 then
    begin
        Healt := Health - damage; //Reduce health
        if Health <= 0 isDead := True;
   end;
end;
This way you also get rid of Died procedure.
4. I also recomend learning about class inheritance: Why? By using class inheritance you make one ancestor class for all units which contains variables common to all of your ingame units (health, postition, unit type, name, description, etc.). And in inherited classes you can add aditional variables which are specific for certain unit types (range for ranged units, armor for armored units, etc.). And becouse Objective Pascal alows you to acces any inherited class by using it paretn class you can acces all the variables declared in parent class without the use of actuall clas of your object.

Man I realy need to finish my Tutorial on working with classes as it would be of great help to you and probably many others.