The Original: Darkwing (TV Show)

When Darkwing is bitten by a giant radioactive spider, he grows four more arms. These hands have a mind of their own and often when fighting Professor Moliarty they are disagreeable. One of them even cranked a handle that lowered the Thunderquack right on Drake.

The only way for Darkwing to get back to normal was for him to find the spider that had bit him and use it to create an antidote. Launchpad, Honker, and Gosalyn find one and rush in the Thuderquack to give the antidote to him, but are stopped by Moliarty. In the end, the radioactive giant spider saves them all before an impending explosion by turning off Moliarty's machine where a lot of radioactivity was happening. The spider returned to his normal size, Moliarty was defeated, and Darkwing was back to normal (or as normal as he gets...).

The Alternate Dimension Arachno-Duck

In the Crisis on Infinite Darkwings story arc, an alternate dimension Darkwing is seen in a similar costume as what Drake once wore (though Drake's was similar in color scheme to Spiderman's red and yellow suit while this one uses Darkwings usual black and purple).

It can be assumed that after this Darkwing was faced with the opportunity to change back to Darkwing (if he had that choice), he did not and so became the new Arachno-Duck. Or perhaps there is another story to this character.

He was one of the Darkwings of the Multiverse whom Negaduck and Magica De Spell were controlling.

He is later seen helping Quiverwing and the rest of the Darkwings of the Multiverse fighting against Negaduck and PaddyWhack. He returned to his world with the rest of the alternates.

Powers (both)

Darkwing had six arms. A lot of them, most specifically the one he called "#6", would not do what he wanted (they would choke him or drop plates, etc.).

He was able to spit streams of Spider silk out of his mouth.

It can be assumed that this alternate Arachno-duck has the same powers and Darkwing once did.