Taillow are found just outside both ends of the Krokka Tunnel (near Fall City and Ringtown). You capture them like any other Pokemon - drawing multiple circles around them. Note that Taillow are weakened by electric blasts from Plusle or Minun.
The first time you can catch Taillow is when Taillow's owner loses it, you can find it in the Lyra Forest, but you cannot find it immediately, first you much catch a Chikorita and Zigzagoon in the area and show it to the owner, he will say that is not his Pokemon and Taillow will roam the forest. Watch out! Taillow is very fast, although didn't put much of a fight for me, circle like mad in the beginning. Note that Taillow can and willrun away after a while.
The second time you have to catch Taillow is when your soon-to-be-partner Pokemon (Plusle or Minun) angered it, your soon-to-be-partner-Pokemon will paralyze it in the beginning so take the advantage! Otherwise its pretty much the same as the other battle, but Taillow will not run away I believe.
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Players can capture a Fearow in Kisara plains in Pokemon Ranger. The Kisara plain is located to the east of Ringtown. Fearow evolves from a Spearow at level 20.
You catch him in the Grassland Capture Challenge in Ringtown, but unfortunately you also have to have beaten the high score at the Capture Arena in Fall City to find him.