Answer:
龍 RYUU, tatsu - dragon
虎 KO, tora - tiger
巨 KYO - large, gigantic
燕 EN, tsubame - swallow
港 KOU, minato - harbour, port
And the winner is 鯉(RI, koi - carp)! They all represent teams in the Baseball Central League!
鯉 Hiroshima Carp
龍 Chunichi Dragons
虎 Hanshin Tigers
巨 Tokyo Giants
燕 Yakult Swallows
港 Yokohama Bay Stars