Why didn't Dumbledore, or Harry, just summon animals to drink the Emerald Potion? That way Dumbledore wouldn't have been weakened considerably by the potion.
As far as I know, there's the Bird-Conjuring Charm (Avis) and the Snake-Conjuring Spell (Serpensortia), either of which could have worked for them.
There are a few problems with doing that.
First and foremost, I think that both Harry and Dumbledore would've seen that as cruel and cowardly.
It's also unclear what limits there are on summoning stuff from within the cave. Voldemort did seem to have thought of the summoning-charm loophole; at a minimum, when Harry tried to summon the Horcrux, it didn't work.
Even if they could summon an animal there, though, it's unclear how they'd be able to control an unwilling animal to a great enough degree to be able to force it to drink the potion (short of one of them physically holding it still and the other one forcibly pouring the potion down its throat, which I can't really see either of them being willing to do). Granted, Harry was a Parselmouth, so he presumably would've been able to exert at least some influence over a snake, but even Parselmouths presumably don't exercise complete control over snakes. Hermoine was also able to set a flock of birds on Ron, but again, that's very different than forcing them to drink multiple large doses of a potion that had vile effects on the drinker.
Finally, this is a little more speculative, but Voldemort seems to have tried to make sure that the person entering would have to weaken themselves, so perhaps he would've somehow included the requirement that only an intelligent being could drink the potion (or had some other similar requirement to try to close this loophole). (Note that I only specify "intelligent being" rather than "human" - clearly, Kreacher was able to drink the potion in spite of being an elf). While the text doesn't directly state that that was the case, the fact that Voldemort thought to make sure that the cup could only be filled from the lake (to make sure that the drinker would have to disturb the Inferni) is an example of the lengths Voldemort went to try to make sure that the drinker would have to weaken themselves as much as possible.
A final possibility: maybe it would have worked, but none of them thought of that. Even Harry and Dumbledore don't seem to have thought of summoning Kreacher, Dobby, or one of the Hogwarts kitchen elves to get them out of there, and Voldemort evidently forgot (or didn't realize) that elves could apparate in and out of the cave. (Harry himself, when he first heard Kreacher's story, was confused as to how Kreacher could've possibly apparated out of the cave - Ron had to point out to him that elves could apparate in and out of Hogwarts when they couldn't, so they were presumably exempt from that restriction).
Dumbledore considered the potion to be highly dangerous and wouldn't have considered allowing any other person or creature to suffer in his place by drinking it. He shrugs off Harry's petition on this basis.
"Why can't I drink the potion instead?" asked Harry desperately.
"Because I am much older, much cleverer, and much less valuable," said Dumbledore. "Once and for all, Harry, do I have your word that you will do all in your power to make me keep drinking?"
(Half-Blood Prince, Chapter 26, The Cave).
Conjured birds may not be as valuable as Harry is, granted. But it seems that Dumbledore also thought that he was the more capable person to face whatever Voldemort had in store. That argument would apply to animals as well.
It would be near-enough impossible, as EJS says, to find an animal that would willingly consent to drinking the potion (even one conjured from thin air). The only way to force it to drink the potion would be by using the Imperius Curse, which Dumbledore was not at all down with.
Again, Dumbledore could have forced an elf or some other creature to drink the potion in his place. But Dumbledore is inherently noble and self-sacrificial and wouldn't have stood for such shenanigans.
"You flatter me," said Dumbledore calmly. "Voldemort had powers I will never have."
"Only because you're too - well - noble to use them."
(Philosopher's Stone, Chapter 1, The Boy Who Lived).
Besides, it's not altogether clear that using conjured beasts would be effective. Dumbledore only considered drinking the potion when every other avenue was closed to him.
"This potion cannot be penetrated by hand, Vanished, parted, scooped up, siphoned away, nor can it be Transfigured, Charmed or otherwise made to change its nature."
Almost absent-mindedly, Dumbledore raised his wand again, twirled it once in midair and then caught the crystal goblet that he had conjured out of nowhere.
"I can only conclude that this potion is supposed to be drunk."
(Half-Blood Prince, Chapter 26, The Cave).
Voldemort had successfully closed all the other loopholes. I doubt that he would leave this one open. I'm sure that if you'd conjured an animal to drink the potion for you, or a fish to swim in and bring it up to you, that that wouldn't have worked. Remember that Voldemort took a great deal of care and devotion in protecting his horcruxes.
That's not to say that a 'real' creature like Fawkes couldn't have done it. But, as per my first point, Dumbledore wouldn't have permitted another creature to suffer in his place.
Dumbledore and, especially Harry, being sensitive personalities could not even think of harming innocent animals for their cause and that's why they didn't even try to summon an animal to drink the potion.
But I am sure Voldemort, being a person without a "single drop of love" in him, would have definitely thought of such a flaw and would have made it such that no one could summon or conjure anything in that cave except a goblet, as conjured by Dumbledore, to drink the potion.
Though one could have easily brought an animal or an elf to drink the potion along with oneself as did Voldemort when he was testing the security measures of his Horcrux, I am certainly sure he was ready to risk that much because of two reasons:
The way he handled his Horcruxes before his disappearance at the Godric's Hollow was even pointed out by Dumbledore in the Half-Blood Prince in reference to Riddle's diary.:
But don't you see, Harry, that if he intended the diary to be passed to, or planted on, some future Hogwarts student, he was being remarkably blasé about that precious fragment of his soul concealed within it.
It suggested that he must have made - or been planning to make - more Horcruxes, so that the loss of his first would not be so detrimental.
It would have ruined the plot. Albus Dumbledore needed to die for Harry Potter to grow up. The potion furthermore revealed the inside of Professor Dumbledore: which was extremely important for Harry Potter when he met Dumbledores brother in the last book.
So it must have been impossible to conjure an animal to drink the potion. We don't really know if Dumbledore has tried it, but we know that he was a smart guy and he tried a lot of spells to get to the Horcrux - without any success. While it might be that he didn't thought of that, I do think that it was prevented by Tom. Just like he made the water vanish when Dumbledore was thirsty after he drank the potion. Tom was really smart.
Conjuring animals or forcing animals to drink the potion wouldn't have worked - at least, that is my opinion. Furthermore: this is a children's book: no animal abuse would have been portrayed.
What would have worked was to force an intelligent life form, like a houseelf, to drink the potion. Just as Tom did. But that would be cruel, so Dumbledore decided not to do it.
