A special thanks to Dota 2 Traders for writing this very helpful guide. A video displaying some of these couriers will be coming later today!
This is a guide that explains exactly how to create a Greevil courier. A greevil courier is hatched from a Greevil egg.
You can imbue your egg with any of five (5) different essences. The three common essences are: Exort (red) Quas (blue), and Wex (Yellow). There is also a rare Unusual essence which adds unusual effects to your greevil and a mythical Shadow Essence which overrides all other essences except for unusual essences. Imbuing your egg with one or a combination of these essences allows you to change both the color and the attributes of your greevil.
As far as we can tell, each color seems to affect a certain attribute while the number of each essence you use seems to affect the adjective of the attribute. Blue affects the nose, yellow the tail, red the horns, green the ears, orange the teeth, and purple the hair. The eyes are always googly and the wings seem to be decided by the highest level of essences in the courier. That is to say that a naked greevil has barely functional wings and a greevil with three (3) of an essence on it will have elaborate wings. The black greevil's attributes are all unique to that color while the white greevil has the maximum description for each attribute as it is made up of all the other colors combined.
The number of unusual essences on your courier also adds unusual effects to the courier. One (1) unusual essence adds a general aura effect. Two (2) unusual essences adds both the aura and a searing eye effect. Finally, three (3) unusual essences adds the aura, the searing eye effect, and also adds a trail. These unusual effects are different for every color of greevil.
Below are examples of the minimum number of essences needed to create the highest tier of each color of greevil as well as the maximum. The maximum also includes the unusual effects of that greevil.
No Essences - Just a Plain Egg
1 Exort Essence, 1 Quas Essence, 1 Wex Essence, 3 Unusual Essences
1 Shadow Essence
1 Shadow Essence, 3 Unusual Essences
3 Quas Essences
2 Exort Essences, 3 Quas Essences, 2 Wex Essences, 3 Unusual Essences
3 Exort Essences
3 Exort Essences, 2 Quas Essences, 2 Wex Essences, 3 Unusual Essences
3 Wex Essences
2 Exort Essences, 2 Quas Essences, 3 Wex Essences, 3 Unusual Essences
3 Quas Essences, 3 Wex Essences
2 Exort Essences, 3 Quas Essences, 3 Wex Essences, 3 Unusual Essences
3 Exort Essences, 3 Wex Essences
3 Exort Essences, 2 Quas Essences, 3 Wex Essences, 3 Unusual Essences
3 Exort Essences, 3 Quas Essences
3 Exort Essences, 3 Quas Essences, 2 Wex Essences, 3 Unusual Essences
3 Exort Essences, 3 Quas Essences, 3 Wex Essences
3 Exort Essences, 3 Quas Essences, 3 Wex Essences, 3 Unusual Essences