たとえば looks like it should be the ば form of a verb, but is that true? I know of the word 例える, but the ば form of this would be 例えれば. The phrase 例えるなら also exists, which would be a cousin if it were really based on some ば form. It's really hard to search for information about this because たとえば is ubiquitous in its use. Grammatically it seems that ば verbs could be used in this way with no problem, so everything in my mind tells me that this is the case. But ultimately I have no real proof of that.
Is たとえば based on a ば verb etymologically, or did it come about by some other mechanism? Is there a たとう from which it originally came?
Note: This question originally suggested for some reason that there was no verb "たとう," a statement that turned out to be completely false.