It is tradition that new riders gain a bird wyvern as their first monstie. And every tradition often has a practical reason for why it was formed.
Multiple reasons can be thought off for this tradition. Bird wyvern eggs are the easiest to get. Bird wyverns are the easiest to train.
Those reasons may be why this tradition was born, however I believe that this tradition may stem from a different reason.
Bird wyverns are social by nature and are born with an instinct to work together and cooperate with their allies.
Working together for the first time with a monster is a difficult hurdle to take, but working together with someone who has a natural talent for cooperation makes this hurdle a lot lower.
I believe that bird wyverns are excellent mentors for teaching us how to work together with others.
I am sure that I can learn a lot from you, and I trust that we will take care of each other well.
-- Giran

About:
Verim was the first monstie hatched by Giran. While Giran still had a few doubts about becoming a Rider, those melted away once he witnessed the young Velocidrome hatching.

In the first few months, Giran wondered how fast Verim's claws were growing, and whether he needed to file them. To measure the growth he painted the nails of Verim in different colors. Verim liked having his nails painted and it has become tradition for them to paint Verim's nails once every month. Verim likes the colour purple the most.

Traits

Favourite colour is purple
Verim is rather sadistic. He enjoys it when he dominates in a fight.
Verim likes to sneak up close on Giras and honk at him to scare him.