~匹 is the counter for small animals - cats, dogs, fish, insects, mice, frogs, and most pets. As a rule of thumb, if the animal is roughly your size or smaller, use ~匹.
~頭 is the counter for large animals - cows, horses, elephants, bears, and the like. It is also the standard counter for livestock and for animals in a zoo, and curiously for butterflies in scientific contexts. If the animal is bigger than a person, use ~頭.
The line between the two is not perfectly strict, and dogs in particular can be counted with either depending on size, but the size rule will serve you well in everyday situations.
~匹 (small animals)
The reading shifts between ひき, ぴき and びき: ぴき after 1, 6, 8, 10, 100 (the small っ numbers), and びき after 3, 1000, 10000 and in 何匹.
| No. | Kana | Kanji |
|---|---|---|
| 1 | いっぴき | 一匹 |
| 2 | にひき | 二匹 |
| 3 | さんびき | 三匹 |
| 4 | よんひき | 四匹 |
| 5 | ごひき | 五匹 |
| 6 | ろっぴき | 六匹 |
| 7 | ななひき | 七匹 |
| しちひき | ||
| 8 | はっぴき | 八匹 |
| 9 | きゅうひき | 九匹 |
| 10 | じゅっぴき | 十匹 |
| じっぴき | ||
| 11 | じゅういっぴき | 十一匹 |
| 12 | じゅうにひき | 十二匹 |
| 13 | じゅうさんびき | 十三匹 |
| 20 | にじゅっぴき | 二十匹 |
| にじっぴき | ||
| 21 | にじゅういっぴき | 二十一匹 |
| 100 | ひゃっぴき | 百匹 |
| 1000 | せんびき | 千匹 |
| 10 000 | いちまんびき | 一万匹 |
| How many | なんびき | 何匹 |
~頭 (large animals)
The reading stays とう; only a small っ appears after 1, 8 and 10.
| No. | Kana | Kanji |
|---|---|---|
| 1 | いっとう | 一頭 |
| 2 | にとう | 二頭 |
| 3 | さんとう | 三頭 |
| 4 | よんとう | 四頭 |
| 5 | ごとう | 五頭 |
| 6 | ろくとう | 六頭 |
| 7 | ななとう | 七頭 |
| しちとう | ||
| 8 | はっとう | 八頭 |
| 9 | きゅうとう | 九頭 |
| 10 | じゅっとう | 十頭 |
| じっとう | ||
| 11 | じゅういっとう | 十一頭 |
| 12 | じゅうにとう | 十二頭 |
| 13 | じゅうさんとう | 十三頭 |
| 20 | にじゅっとう | 二十頭 |
| にじっとう | ||
| 21 | にじゅういっとう | 二十一頭 |
| 100 | ひゃくとう | 百頭 |
| 1000 | せんとう | 千頭 |
| 10 000 | いちまんとう | 一万頭 |
| How many | なんとう | 何頭 |
Check out other counters.