Help with Eidolon Build

[Duna the Explorer of Indol]

I already have my perfect eidolon built out on a piece of paper but I have run into two questions:
1) If it's a serpentine and it has a tail slap attack would giving it a poisonous tail add an attack or replace the tail slap?
2) If I give it gills it can breathe underwater, would it make it unable to breathe air like a fish, or can it do both?
Thanks a lot on this.
-Dunali

[Tirq]

1) You have to have another tail for that.
2) You breath like the military (On Land, In Air, By Sea)

[darth_borehd]

Always go for evolutions that give primary attacks instead of secondary ones. Check to see if you can get a primary attack form instead. Maybe a Gore attack?

What Tirq said. Unless stated otherwise, evolutions like gills don't come with drawbacks like that.

[Archaeik]

darth_borehd wrote:

Always go for evolutions that give primary attacks instead of secondary ones. Check to see if you can get a primary attack form instead. Maybe a Gore attack?

What Tirq said. Unless stated otherwise, evolutions like gills don't come with drawbacks like that.

Serpentine automatically comes with a tail slap attack.

To the OP, I'll assume you're talking about a Sting and giving it Poison.

I'm not aware of a bestiary creature that has both(tail slap and sting) on the same tail, but I know of nothing RAW that precludes it. I see no reason to disallow it, just like bite and gore being available on the same head.

IOW, ask your GM.