Beastiary : my own version

Leprechauns

Leprechauns, also known as Pucks or children of Tyche, are a breed of Seelie who manipulate the element of luck. They have gained a reputation for their greed and avarice after using their abilities over luck to gain fortune from humans via gambling and controlling the probability of the games to always win.

Leprechauns have a very subtle shift as they gain a faint golden glow than circles round the iris when they use their powers. Other than that they are visually indistinguishable from human.

Society:
These creatures rarely socialise with other types of Seelie, some avoid even their own. Despite this they are known to occasionally meet in small or large colonies in times of danger or celebration. Other Seelie sometimes look down on leprechauns because of their greed but rarely say it as leprechauns tend to hold grudges and can conjure great misfortune to their enemies.

Powers and abilities:
Like all supernaturals leprechauns have their own special traits and abilities.

Longevity: Like all Seelie they have an extended life span capable of living hundreds, maybe even thousands, of years.
Superhuman strength: Despite mythology leprechauns are not smaller or weaker than humans an are in fact very strong, however they are not as strong as some other supernaturals.
Superhuman agility: They can move quicker and easier than what is humans possible and have a more acute reaction time. 
Luck manipulation: Leprechauns can control probability and use this to increase the luck of themselves or others to grant them more success in their risks however they can also do the opposite using luck as a weapon by cursing others causing them to have misfortune. Luck however is not completely under the users control as they can make the probability of something happening much greater however their is always a chance of failure.
Luck enchantments: They have the ability to magically enchant objects so they that they generate an aura of either positive or negative luck.

Notable enchanted objects: Although they could theoretically enchant practically any tangibly solid object these are the most common objects they tend enchant.

Shillelaghs: Thick walking-stick like weapons usually made of blackthorn or oak. These are often enchanted so that they can fire waves of misfortune at an enemy, which could cause the environment to effect them such as rumble falling on them. They could as be enchanted so that they are more likely to hit an enemy's more vulnerable areas once swung or the enemy will be in the right place at the right time so they are open to being hit.
Coins: Coins are often great conducts of luck and will often easily generate positive or negative, depending on the enchantment, luck to those in possession of the coins. They are given as either a reward or punishment to a leprechauns associates. A leprechaun could also wear a certain number of coins on themselves to either increase, decrease or balance out their luck.
Guns: Similar to Blacksmiths Leprechauns can be found of firearms, however they don't use them as much as their technological counterpart, they often enchant these guns so they are less likely of missing despite the users aim and can cause the user to be able to make near impossible shots.

Weaknesses:
Like all supernaturals Leprechauns can be weakened and killed under the right circumstances.

Excessive Physical trauma: They can be killed by to much physical injury and can be harmed by mortal weapons.
Iron: Iron burns more pure blooded Seelie to the touch but is described as more of a freezing feeling rather than an burn hence the term "cold iron". Only pure iron affects them so steel and other iron alloys won't affect them.
Rowan wood: Seelie magic is weakened and doesn't work on objects made of rowan wood.