I'm browsing though the pet trades tab again and see people trying to buy pets for a very expensive price. These pets have combo abilities? 

I barely ever give vermillion to a pet unless it's a deviant, are combo ability rare or is it just certain combos?

I just vermillion'd a deviant pet and it gave me counterstrike, what is it for?  

Thanks!

it makes your pet able to do a atack  that deald mass damage 

i have one on a Lev 50 pet gp3 called Lightning Strike when it uses it i can deal up to 400 hit points that will wipe any pet out in one shot

Counterstrike is an ability that gives a chance for your pet to deal a little extra damage if they dodge the opponent's attack. Not to be confused with an elemental attack combo such as Lightning Strike, Acid Spit, or Fire Breath, which deals 150% of normal attack damage when using Bite.

Arguably the elemental attack combo is rarer, since both the genes it requires are recessive to nearly every other, while Counterstrike requires a universally dominant gene, making it easier to breed. Counterstrike comes from the combination of Quickmind and Agile Paws, while elemental attack comes from Fire Gland/Acid Gland/Steel Beak and Spitting Muscle/Electric Gland, depending on species. (Also, lykos only require one gene, Ice Breath, to gain elemental attack, making it the easiest to get.)

Personally, the presence of a combo ability does not dramatically increase a pet's value to me unless its other genes are good as well. Some people put more stock in them than others.

I like Ice breath and Ice aura as a combo. Ice aura is similar to Magic horn, making your opponent miss a turn, and Ice breath is a great attack, very powerful. It's two for the price of one. Pity I gave up my best female for an etelia. Although the etelia's Blizzard attack combined with Ice breath is pretty good, it isn't gp 3.