So let's think a little about how to use reaction dice to make animal encounters interesting.
BECMI has the following reaction dice table:
4-6 Aggressive (roll again -4 next round)
7-9 Cautious (roll again next round)
10-11 Neutral (roll again +4 next round
This works nice as a quick and dirty method but it could be elaborated on, particularly if you want to make interesting encounters with animals that are magical but not normally predatory on humans. It also occurs to me that you can actually communicate something about the totemic nature of animal species through the reaction dice table, or even change it on an encounter-by-encounter basis.
So for the sake of continuity let's think about shags. (Phwoar.) I like this idea of shags as being a kind of elemental spirit of the shore: a sort of sprite which is produced from the collision between land, wind and sea. A beast of stone and salty water and gale force winds.
Shags are never going to attack the PCs, nor really be "aggressive" towards them as you'd normally understand it. So if you have "shag" in an encounter table, you need to have the reaction dice mean something different if "attack" or "aggressive" come up. And you may as well do it for the other results too (because what exactly does it mean for a shag to be friendly? (Nice and slow, if you want to innuendo done for you)). For example:
Attack result: roll a d6 and consult the following:
1 - The shags summon a minor earth, water or air elemental (3 HD) to protect them
2 - The shags cause a wave to swamp the shoreline, causing PCs to fall over on a failed save versus paralyzation, which results in d3 hp damage in bruises, cracked bones/skulls
3 - The shags are the servants of a coastal warlock who collects flotsam and jetsam and lives in a nearby cave. He comes to attack the PCs and will arrive in d6 turns, clambering over the rocks with alarming speed.
4 - The shags croak aggressively and alert the other seabirds nearby, which bombard the PCs with their dung (PCs stink for the rest of the week unless they get a thorough bath, losing all surprise rolls automatically)
5 - The shags use their power over the earth to cause rocks to fall from the nearby cliff face: roll a d6 for each PC; on a roll of 1 her or she is hit for 3d6 damage
6 - The shags flee into the water and, through their magical power, cause the PCs to want to pursue; each must successfully save vs. magic or chase after them into the deep and cold sea
You get the idea - some of these may be a bit too tough.
Hopefully this communicates something of what I mean. The PCs encounter a group of shags at the bottom of a sea cliff. They get an "attack" result on the reaction dice and the shags summon a minor air elemental. They are revealed to have this power through the reaction table; the DM didn't predetermine this - he just created the potential for it to happen and the dice instantiate it. Maybe all shags have this ability or maybe it is just these ones, and the DM has the chance to think some more about his world. Similarly, the '3' result introduces some new opportunity to the adventure - probably the list could do with more such results - again deepening the DM (and PCs') sense of a living world.
There's no reason why you have to stop at animals with this, of course. Think of what you could do with, say, orcs. A list of potential 'friendly' results might be:
1 - The orcs offer money in return for the PCs attacking the nearby lair of a troll
2 - One of the orcs is a half-orc who bears a weird resemblance to a family member of one of the PCs
3 - The orcs are escapee slaves from a local mage and throw themselves at the mercy of the PCs
4 - The orcs are from a certain tribe who try as much as possible to dress, talk and act like humans, and want to 'ape' the PCs in precise detail...
&c. &c. The game world gets fleshed out simply through the PCs meeting things.