Because of the 1 toughness, I would be inclined to think Goblin, Elf, or some sort of elemental. Goblin would be not very intresting. Elf seems like the most logical... they already "splash" with black so its not too out of elf context to splash into red. Also there are examples of weak elves with trample.
Flavor 1 -
Granit-Skin Elf
Creature - Elf
"Don't worry so much about speed... you must focus on your follow though" - Grandmaster Athyresh
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I'm partial to making it an elemental. This will capture the ability for the creature to permiate other creatures.
Flavor 2 -
Gale Force Elemental
"Never underestimate the strength of the Westward Gale... I'd like to see you make an oaktree grow sideways!"
-- Groth'mendor, Dwarven Veteran
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Inspiration for a cycle.
I know its easy to get into the mindset of "OMG LEST MAKE THIS A CYCLE!!" But I think this could be a fairly interesting 5 card cycle.
Eidolon -
1) A phantom; an apparition.
2) An image of an ideal.
Also -- From Greek Mythology: A human with animal parts
Starting with R -> G which gives you Trample if you have Haste. In other words Haste is granted trample. Extrapolating that would mean you'd have the following:
R -> G [Haste => Trample] Eidolon of the Gale (2/1)
G -> W [Tample => Vigilance] Eidolon of Iron Roots (1/2)
W -> U [Vigilance => Flying] Eidolon of Righteousness (1/2)
U -> B [Flying => Fear] Eidolon of Nightmares (1/1)
B -> R [Fear => Haste] Eidolon of Recklessness (1/1)
I think each should cost just 2 mana (one of each color). This seems realy neat to me, because you could chian thier abilites together, so for example If I had the RG and the GW ones in play, I could play a Raging Goblin with Haste and it would gain both Trample and Vigilance. Seems cool to me.
EDIT: OMG I totally misread the original card as "All creatures with haste gain trample." My flavors don't really make sense the other way around

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