You'll have to cast a few creatures with greater power than one, then two, and maybe even three, but once you do, Pollywog Prodigy is going to start drawing cards like it's going out of style
On top of that, you can spend 5 mana to pseudo-tutor for more creatures with activated abilities by milling the top five cards from your library
Cards used to respond must either be Instant or have the Flash keyword
Plague Mare is obviously not as good, as its a 2/2 instead of a 3/3 with first strike, and it doesnt deal damage to the opponent or planeswalkers they control, but it does have some upside
The most valuable of the four, however, is the Alternate Full Art version with dreamy, unsettling artwork by Midori Harada that delivers an astounding price of $582