Is there an old way of pronouncing 牠? I remember learning to pronounce it as "tuo" when I was in primary school, which was more than 40 years ago. I know it is pronounced as "ta" now.
I think it's an old pronunciation. I remembered this pronunciation appeared in a classic poem composed approximately in Tang dynasty.
The note says the correct pronunciation is "tuo", which rhymes with "罗","多" and "么". According to the poem's pattern,"他" should rhyme here.
In modern usage, I don't sound "tuo" in any case. "ta" is the only correct pronunciation. In order to rhyme, I'll read some characters with old pronunciations when reading classic poems.
Such a phenomenon is called "古音异读" (lit. special ancient pronunciation), however, while the languages is evolving as time flies, some special pronunciations have been ignored by the public, and thus get lost.
Some other examples for "古音异读". The fomer pronunciation is "modern pronunciation", and the latter is the "special ancient pronunciation".
Sometimes I'm still confused when reading these characters. A typical one is "骑", when I learned it in a famous verse "一骑红尘妃子笑", I was taught the correct pronunciation was "jì" which means "a rider on a horse"; however, now in the dictionary, the pronunciation for this meaning is also qí.
I can't find direct evidence and reference, but according to the book 字源谈趣, I think you're right. And for the further explanation:
This is a picture about Chinese character evolution of "它". Its original meaning is "snake".
And at the Qin Dynasty(from 221 to 206 BC), as shown as picture I, "也" was evolved from "它", and their ancient pronunciation were both "tuo1".
And "牠" appeared from early vernacular in modern times, using "牛" as ideograms, using "也" as phonogram. And "也"'s ancient pronunciation was "tuo1", so "牠"'s ancient pronunciation shoule be "tuo1" too. And nowadays it's as "它"'s variant character and rarely be used.