Mine is well... kinda odd depending on what you think about it.

Yeah, mine’s Pharynx.

Celestia/Luna, also possibly Sunset.

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Ah. Interesting.

CMC, Pinkie Pie, Starlight, Fluttershy, Sugar Belle, and Vapor Trail

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Hm... Vapor Trail. I’ve never seen her as a victim before.

First, I'd like to say that I prefer it when the one being tickled enjoys it. Unfortunately, we don't know if that's the case for most characters.

Okay, for ponies, I'm limiting myself to two "adults" and two children from each of the three common pony types, because if I don't, I might as well say "every filly and young mare", because there are so many adorable possibilities. This is by no means exhaustive.

If you don't recognize a name, there's a wiki, look it up.

Pinkie seems like she'd be fun to tickle... maybe my favorite background pony, Diamond Mint… Lyra, particularly when she sits with her belly left vulnerable... Flitter (though not Estee's version!)... Helia… Octavia...

And among the fillies, there's of course Apple Bloom and Sweetie Belle, as well as Zipporwhill, Candy Caramel Tooth (mainly on the strength of that one scene from "Apple Family Reunion" where she lets Apple Flora tickle her), Dinky Doo, and Tornado Bolt.

Ocellus and Silverstream could be fun, too... I want to list Smolder, but... I don't know how she'd react.

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First, I'd like to say that I prefer it when the one being tickled enjoys it. Unfortunately, we don't know if that's the case for most characters.

That’s why we have headcanons. I headcanon that Pharynx (yes, that Pharynx) enjoys being tickled, but he’s too much of a grumpy tsundere to admit it.

I also like Sombra as a victim.

I dunno, I like seeing ticklish stallions.

I like seeing Twilight get it. And, also, Smolder, Ocellus, Starlight, Apple Bloom, & Sunset.

Pinkie Fluttershy and Rainbow