public class EmailController {
    @AuraEnabled 
    public static List<EmailTemplate> getEmailTemplate(){
        return [SELECT Id,Body,DeveloperName FROM EmailTemplate];
    }
}

<aura:component controller="EmailController">
    
    <aura:attribute name="options" type="List" default=""/>
    
    
    <aura:handler name="init" value="this" action="{!c.doInit}"/>
    
    
    <lightning:combobox name="Email Templates" label="Email Templates" value="Email Templates" placeholder="Select Email Templates" 
                        options="{! v.options }" onchange="{! c.handleChange }"/>
    
    
    
</aura:component>

({
    doInit : function(component, event, helper) {
        var action = component.get("c.getEmailTemplate");
        
        action.setCallback(this, function(response) {
            var state = response.getState();
            if (state === "SUCCESS") {
                let resp =response.getReturnValue() ;
                let lables= [] ; 
                resp.forEach(function(key) {
                    lables.push({"label":key.DeveloperName ,"value":key.DeveloperName});
                });
                component.set("v.options", lables);
                //   cmp.set("v.contactList", response.getReturnValue());
            }
            
        })
        
        $A.enqueueAction(action);
        
    },
    
    handleChange: function(component, event, helper){
        
        
    }
    
})