veins
I think what you're referring to are the gills.?
They are red because you can see the colour of the red blood through the thin walls of the gills. The gills carry the oxygenated blood into and around the fish just as your lungs do for you.
Yes, the "red thing" is actually called a membrane. Bettas will flare out their gills and membranes when he is feeling threatened, especially by another Betta fish.
Myoglobin, a Protein which is red in pigment
red
they are bloody
The gills of a healthy fish are bright red; this is because blood is flowing through them.
The red stuff in steak is called myoglobin, a protein that stores oxygen in muscle cells. When exposed to oxygen, myoglobin turns red, giving the steak its red color.
frogs with gills a frogs with gills are called a tadpole
Roosters are not an aquatic species and therefore do not have gills. If you are referring to the red skin hanging under the beak, they are called wattles and they help cool the chicken during hot weather.
Those are its gills (e.g. for breathing). Salamanders such as the Axolotl have external gills, unlike internal gills found in fish.
JB gills from JLS is dating a girl called chloe