This "1861 Buffalo" Quarter is an especially appealing struck-thru grease Quarter, both for its eye appeal and subject matter. The buffalo, or bison, appears to be coming out from the mist next to several wild native sunflowers under the year 1861. This design is the result of a struck-thru grease error.
Struck-thru errors occur when some material is lodged between a die and the planchet between striking, resulting in an imprint of the material on the coin. A relatively common struck thru material is die grease, also known as die fill, which can be a mixture of lubricant, metal dust, and dirt. This tends to change the visual design of the coin by obscuring design elements on the obverse and/or reverse.
While struck-thru grease coins are relatively common in general, it can be challenging and rewarding to find designs where the struck-thru portion results in eye appealing designs, giving the coin a very dramatic look.
In this particular case, the reverse is especially noteworthy for leaving only the design elements of the bison and flowers along with the year 1861. The state name "KANSAS" and issue year "2005" are obscured along with part of the "E. PLURIBUS UNUM" motto.
I normally don't focus on struck-thru error coins since most are common and of little value, but this one jumped out at me. It's a perfect example of a major struck-thru error creating a stunning look that's different than the intended design.