slimytrout: This looks like a rare and reads like a rare. Negative prowess is something different, and appropriate for black. I'm not sure why this is a mythic rare.
Henlock: Elegant and beautiful. Could use a line or two of flavor text.
netn10: Reminds me of
Lightning Skelemental. Storm players would love you. I think that this is just right in terms of power, but could be dangerous in big formats.
Venedrex: This card doesn't work as worded. The final sentence in the ability should use a delayed triggered ability, not a replacement effect. I do like how the cards are supposed to return to your graveyard by virtue of them being cast, then countered. Unique, but needs better execution.
kwanyeegor-ii: This does a lot for one mana, but it doesn't feel that cohesive. Yes enchantments will trigger prowess, but being pumped by life gain is a little random I feel. I'm not sure this is comparable to existing options for one-mana prowess creatures.
MonoRedMage: This could be a bomb in Limited, provided you have enough cheap cantrips to keep fields cleared. Maybe could use an extra point of power?
Legend: Do the rules as they stand support keyword counters other than evergreen keywords? This is a cool expansion of that idea, but unfortunately I cannot vote for this as it technically does not meet the challenge.
1) Henlock
2) netn10
3) MonoRedMage