ホモテナシ

筆

When looking at a brush, you're interested in:
- 尖(せん): Is it really pointy?
- 斉(さい): Do the hairs stay together when flattened?
- 円(えん): Is the body nice and round?
- 健(けん)): Does it spring back into its original shape?
- (and also ink retention)
Size matters (the shorter, the easier to control; but longer gives more expressivity)
There are brushes for writing 漢字 and for writing かな (or 仮名). The latter is often made from weasel hair (イタチ毛) or a Siberian weasel (called コリンスキー, for some reason).
Brushes are made from different hairs:

毛	Reading	Animal	Character	Suitable
羊毛	ようもう	Goat hair (despite using 羊)	Very soft, difficult to control; large expressive range	行書・草書
鼬毛・イタチ毛	いたちげ	Weasel hair (sometimes also 狼毛, which here doesn't mean wolf)	Fine point, strong spring, responsive	仮名・細字・楷書
コリンスキー		Siberian-weasel tail hair (Mustela sibirica), technically a sub-type of イタチ	Premium weasel hair; fine and relatively long; excellent point, elasticity	仮名・細字・楷書
馬毛	うまげ	Horse hair; different parts have different properties; tail, f.e., is very resilient (called 天尾)	Generally strong and resilient; can be coarse; forceful, clear strokes	楷書
狸毛	たぬきげ	Raccoon!	Firm tip, very strong resilience; good when the point needs authority	楷書
兎毛・紫毫	うさぎげ・しごう	Rabbit; 紫毫 traditionally refers to dark rabbit hair	Sharp, resilient, rather delicate; historically prestigious	細字
玉毛・猫毛	たまもう・ねこげ	Cat hair	Very fine, soft, with distinctive sticky/supple elasticity	仮名
猪毛	いのししげ	Boar hair	Very stiff, rough, produces deliberately vigorous/rough lines	漢字
兼毫・兼毛	けんごう・かねもう	Blend