MArcomage

I think it's clear that this card is way too specific. How about something like: "Discard highest cost attack card in enemy hand. If discarded card was Dragon, gain cost of discarded card."?

(And maybe if we did that, the name should be changed to something like "Ardent Defender", or "Heroic Knight" or something)

I agree. However I was thinking about adding soldier and banish keyword. Your idea is also quite good as well.

Add Horde?
Discard highest rarity Unliving, Undead, Horde or Dragon from opponent's hand

Think I just encountered a bug with this one; it's supposed to discard highest rarity, but when I played it just recently, it discarded Skeleton Warriors (common), when my opponent had a Vampire (uncommon) in hand, which he played immediately thereafter.

Screenshot: http://i.imgur.com/eny2M.jpg

EDIT: Scratch that, actually; just finished that game and consulted the replay. Turns out that Monster Slayer discarded the common Skeleton Warriors, and a Vampire was drawn as a result.

Add Beast?