From Sonic Retro
Animals are various fluffy forest critters imprisoned by Dr. Eggman for use as 'organic batteries' in his badnik army. They can be freed by destroying badniks individually, and released en masse by popping open the Egg Capsules which appear at the end of many zones.
In this game there are always two animals per stage, one per badnik released and a large number of them released from a capsule at the end of Act 3, the latter with an exception in Scrap Brain Zone, which has no capsule.
In this game there are always two animals per stage, as in Sonic the Hedgehog (16-bit), but they only exist in the capsule at the end of Act 3. Scrap Brain Zone and Sky Base Zone lack the animal capsule.
In this game there are always two animals per stage like in Sonic the Hedgehog (16-bit), one per badnik released and a large number of them released from a capsule but instead of Act 3 at the end of Act 2. There is also an unnamed animal in Mystic Cave Zone who looks like a rat. Sky Chase Zone and Wing Fortress Zone have no animal capsule at the end. If playing Knuckles in Sonic 2, the Flickies in Aquatic Ruin Zone and Casino Night Zone are red instead of the usual blue.
In this game there are always two animals per stage like in Sonic the Hedgehog (16-bit) but as in the previous title, the animals are only present at the end of Act 3 capsule, which is different from the norm: an Eggman ball floating around that needs to be hit. Scrambled Egg Zone and Crystal Egg Zone have no animal capsule.
In this game there are always two animals per stage, as in Sonic the Hedgehog (16-bit), one per badnik released and a large number of them released from a capsule at the end of Act 2 like in Sonic the Hedgehog 2 (16-bit). As a curiosity, the Flickies in Launch Base Zone with Knuckles are red instead of the usual blue with Sonic.
As expected, this game follows the formula of Sonic the Hedgehog 3. Hidden Palace has no animals whatsoever and neither Sky Sanctuary or Death Egg Zone has animal capsules - though the former Zone does see the diminutive creatures fleeing from Eggrobos inside the level's towers.
There are 15 different animals in Sonic Adventure, found both inside Badniks and capsules at the end of the level like in Sonic the Hedgehog (16-bit) or any game that follows this format but they serve a purpose this time around. Colour coded according to their stats for five different categories, collecting, bringing them to a Chao Garden and giving them to a Chao will raise its stats according to the equivalent of the animal collected. Every level has five different animals, regardless of the character playing, three of them matching the level’s theme (for example, Emerald Coast has all the swimming types, Windy Valley have all the flying types and so on), another from some other specific stat and finally a last from the random category. When released, there’s a green light sphere around them, which disappears shortly. When in a Chao Garden, there’s a sparkle around them that disappears after giving it to a Chao. Once its sparkle is gone, the animal itself will also disappear after a while.
Sonic Adventure 2
There are 21 different animals in Sonic Adventure 2. Eight of them come from Sonic Adventure; the rest of them are new, and they serve the same purpose as the previous game, for raising Chao according to the stats they possess. They’re not found in capsules anymore, since the game lacks them, but they can still be found inside Badniks. Addictionally, animals can be found inside the second and third Chao containers, and in several locations in each stage.
There are two new categories to add to the previous five and there are always five specific animals per stage. Four of these animals are common (Running, Flying, Power and Swimming), found with nor restrictions and in the second Chao container. The fifth is always a rare or special animal (Ghost and Legendary, with the exception of the Bat which is treated as common) and they are found either in hidden or difficult to reach locations and in the third Chao container.
The number of animals hidden in a stage (see the stage’s entrance for their location) is always the same (20 when playing as Sonic or Shadow, 15 as Tails or Eggman, and 10 as Knuckles or Rouge). Collecting all of them will award an extra-life. There are several levels where there are more animals than they should (one notable case is Security Hall); Cannon's Core doesn't seem to have a proper tally and Green Hill doesn't have any wandering animals at all.